SERVER-33179 Remove featureCompatibilityVersion 3.4

19 files changed, 105 insertions, 432 deletions

diff --git a/jstests/noPassthroughWithMongod/isMaster_feature_compatibility_version.js b/jstests/noPassthroughWithMongod/isMaster_feature_compatibility_version.js
index 0c6fc4de768..ab22bfb5f6c 100644
--- a/jstests/noPassthroughWithMongod/isMaster_feature_compatibility_version.js
+++ b/jstests/noPassthroughWithMongod/isMaster_feature_compatibility_version.js
@@ -54,15 +54,4 @@
     assert.eq(res.minWireVersion, 0, tojson(res));
     assert.lt(res.minWireVersion, res.maxWireVersion, tojson(res));
 
-    // When the featureCompatibilityVersion is equal to the deprecated downgrade version, running
-    // isMaster with internalClient returns minWireVersion + 1 == maxWireVersion.
-    // TODO(SERVER-33179) remove this section once fCV 3.4 is removed.
-    const downgradeVersion = "3.6";
-    const deprecatedDowngradeVersion = "3.4";
-    assert.commandWorked(adminDB.runCommand({setFeatureCompatibilityVersion: downgradeVersion}));
-    assert.commandWorked(
-        adminDB.runCommand({setFeatureCompatibilityVersion: deprecatedDowngradeVersion}));
-    res = adminDB.runCommand(isMasterCommand);
-    assert.commandWorked(res);
-    assert.eq(res.minWireVersion + 2, res.maxWireVersion, tojson(res));
 })();
diff --git a/src/mongo/db/catalog/catalog_control.cpp b/src/mongo/db/catalog/catalog_control.cpp
index fad8950aed0..7a8ccfdb0b1 100644
--- a/src/mongo/db/catalog/catalog_control.cpp
+++ b/src/mongo/db/catalog/catalog_control.cpp
@@ -152,13 +152,11 @@ void openCatalog(OperationContext* opCtx) {
                                     << collName);
 
             auto uuid = collection->uuid();
-            // TODO (SERVER-32597): When the minimum featureCompatibilityVersion becomes 3.6, we
-            // can change this condition to be an invariant.
-            if (uuid) {
-                LOG(1) << "openCatalog: registering uuid " << uuid->toString() << " for collection "
-                       << collName;
-                uuidCatalog.registerUUIDCatalogEntry(*uuid, collection);
-            }
+            invariant(uuid);
+
+            LOG(1) << "openCatalog: registering uuid " << uuid->toString() << " for collection "
+                   << collName;
+            uuidCatalog.registerUUIDCatalogEntry(*uuid, collection);
 
             // If this is the oplog collection, re-establish the replication system's cached pointer
             // to the oplog.
diff --git a/src/mongo/db/commands/apply_ops_cmd.cpp b/src/mongo/db/commands/apply_ops_cmd.cpp
index e7dc2a5a6fa..34010f2aa3f 100644
--- a/src/mongo/db/commands/apply_ops_cmd.cpp
+++ b/src/mongo/db/commands/apply_ops_cmd.cpp
@@ -163,13 +163,6 @@ OplogApplicationValidity validateApplyOpsCommand(const BSONObj& cmdObj) {
 
             bool opHasUUIDs = operationContainsUUID(opObj);
 
-            if (serverGlobalParams.featureCompatibility.getVersion() ==
-                ServerGlobalParams::FeatureCompatibility::Version::kFullyDowngradedTo34) {
-                uassert(ErrorCodes::OplogOperationUnsupported,
-                        "applyOps with UUID requires upgrading to FeatureCompatibilityVersion 3.6",
-                        !opHasUUIDs);
-            }
-
             // If the op uses any UUIDs at all then the user must possess extra privileges.
             if (opHasUUIDs && ret == OplogApplicationValidity::kOk)
                 ret = OplogApplicationValidity::kNeedsUseUUID;
diff --git a/src/mongo/db/commands/feature_compatibility_version.cpp b/src/mongo/db/commands/feature_compatibility_version.cpp
index 1781a956435..afa789dd60e 100644
--- a/src/mongo/db/commands/feature_compatibility_version.cpp
+++ b/src/mongo/db/commands/feature_compatibility_version.cpp
@@ -89,17 +89,14 @@ StatusWith<ServerGlobalParams::FeatureCompatibility::Version> FeatureCompatibili
             }
 
             if (elem.String() != FeatureCompatibilityVersionCommandParser::kVersion40 &&
-                elem.String() != FeatureCompatibilityVersionCommandParser::kVersion36 &&
-                elem.String() != FeatureCompatibilityVersionCommandParser::kVersion34) {
+                elem.String() != FeatureCompatibilityVersionCommandParser::kVersion36) {
                 return Status(ErrorCodes::BadValue,
                               str::stream() << "Invalid value for " << fieldName << ", found "
                                             << elem.String()
                                             << ", expected '"
                                             << FeatureCompatibilityVersionCommandParser::kVersion40
-                                            << "', '"
-                                            << FeatureCompatibilityVersionCommandParser::kVersion36
                                             << "' or '"
-                                            << FeatureCompatibilityVersionCommandParser::kVersion34
+                                            << FeatureCompatibilityVersionCommandParser::kVersion36
                                             << "'. Contents of "
                                             << FeatureCompatibilityVersion::kParameterName
                                             << " document in "
@@ -130,48 +127,17 @@ StatusWith<ServerGlobalParams::FeatureCompatibility::Version> FeatureCompatibili
         }
     }
 
-    if (versionString == FeatureCompatibilityVersionCommandParser::kVersion34) {
-        if (targetVersionString == FeatureCompatibilityVersionCommandParser::kVersion40) {
-            return Status(ErrorCodes::BadValue,
-                          str::stream() << "Invalid state for "
-                                        << FeatureCompatibilityVersion::kParameterName
-                                        << " document in "
-                                        << FeatureCompatibilityVersion::kCollection
-                                        << ": "
-                                        << featureCompatibilityVersionDoc
-                                        << ". See "
-                                        << feature_compatibility_version::kDochubLink
-                                        << ".");
-        } else if (targetVersionString == FeatureCompatibilityVersionCommandParser::kVersion36) {
-            version = ServerGlobalParams::FeatureCompatibility::Version::kUpgradingTo36;
-        } else if (targetVersionString == FeatureCompatibilityVersionCommandParser::kVersion34) {
-            version = ServerGlobalParams::FeatureCompatibility::Version::kDowngradingTo34;
-        } else {
-            version = ServerGlobalParams::FeatureCompatibility::Version::kFullyDowngradedTo34;
-        }
-    } else if (versionString == FeatureCompatibilityVersionCommandParser::kVersion36) {
+    if (versionString == FeatureCompatibilityVersionCommandParser::kVersion36) {
         if (targetVersionString == FeatureCompatibilityVersionCommandParser::kVersion40) {
             version = ServerGlobalParams::FeatureCompatibility::Version::kUpgradingTo40;
         } else if (targetVersionString == FeatureCompatibilityVersionCommandParser::kVersion36) {
             version = ServerGlobalParams::FeatureCompatibility::Version::kDowngradingTo36;
-        } else if (targetVersionString == FeatureCompatibilityVersionCommandParser::kVersion34) {
-            return Status(ErrorCodes::BadValue,
-                          str::stream() << "Invalid state for "
-                                        << FeatureCompatibilityVersion::kParameterName
-                                        << " document in "
-                                        << FeatureCompatibilityVersion::kCollection
-                                        << ": "
-                                        << featureCompatibilityVersionDoc
-                                        << ". See "
-                                        << feature_compatibility_version::kDochubLink
-                                        << ".");
         } else {
             version = ServerGlobalParams::FeatureCompatibility::Version::kFullyDowngradedTo36;
         }
     } else if (versionString == FeatureCompatibilityVersionCommandParser::kVersion40) {
         if (targetVersionString == FeatureCompatibilityVersionCommandParser::kVersion40 ||
-            targetVersionString == FeatureCompatibilityVersionCommandParser::kVersion36 ||
-            targetVersionString == FeatureCompatibilityVersionCommandParser::kVersion34) {
+            targetVersionString == FeatureCompatibilityVersionCommandParser::kVersion36) {
             return Status(ErrorCodes::BadValue,
                           str::stream() << "Invalid state for "
                                         << FeatureCompatibilityVersion::kParameterName
@@ -203,26 +169,6 @@ StatusWith<ServerGlobalParams::FeatureCompatibility::Version> FeatureCompatibili
     return version;
 }
 
-void FeatureCompatibilityVersion::setTargetUpgrade_DEPRECATED(OperationContext* opCtx) {
-    // Sets both 'version' and 'targetVersion' fields.
-    _runUpdateCommand(opCtx, [](auto updateMods) {
-        updateMods.append(FeatureCompatibilityVersion::kVersionField,
-                          FeatureCompatibilityVersionCommandParser::kVersion34);
-        updateMods.append(FeatureCompatibilityVersion::kTargetVersionField,
-                          FeatureCompatibilityVersionCommandParser::kVersion36);
-    });
-}
-
-void FeatureCompatibilityVersion::setTargetDowngrade_DEPRECATED(OperationContext* opCtx) {
-    // Sets both 'version' and 'targetVersion' fields.
-    _runUpdateCommand(opCtx, [](auto updateMods) {
-        updateMods.append(FeatureCompatibilityVersion::kVersionField,
-                          FeatureCompatibilityVersionCommandParser::kVersion34);
-        updateMods.append(FeatureCompatibilityVersion::kTargetVersionField,
-                          FeatureCompatibilityVersionCommandParser::kVersion34);
-    });
-}
-
 void FeatureCompatibilityVersion::setTargetUpgrade(OperationContext* opCtx) {
     // Sets both 'version' and 'targetVersion' fields.
     _runUpdateCommand(opCtx, [](auto updateMods) {
@@ -346,15 +292,9 @@ void FeatureCompatibilityVersion::updateMinWireVersion() {
             spec.outgoing.minWireVersion = LATEST_WIRE_VERSION;
             return;
         case ServerGlobalParams::FeatureCompatibility::Version::kFullyDowngradedTo36:
-        case ServerGlobalParams::FeatureCompatibility::Version::kUpgradingTo36:
-        case ServerGlobalParams::FeatureCompatibility::Version::kDowngradingTo34:
             spec.incomingInternalClient.minWireVersion = LATEST_WIRE_VERSION - 1;
             spec.outgoing.minWireVersion = LATEST_WIRE_VERSION - 1;
             return;
-        case ServerGlobalParams::FeatureCompatibility::Version::kFullyDowngradedTo34:
-            spec.incomingInternalClient.minWireVersion = LATEST_WIRE_VERSION - 2;
-            spec.outgoing.minWireVersion = LATEST_WIRE_VERSION - 2;
-            return;
         case ServerGlobalParams::FeatureCompatibility::Version::kUnsetDefault36Behavior:
             // getVersion() does not return this value.
             MONGO_UNREACHABLE;
@@ -365,16 +305,13 @@ void FeatureCompatibilityVersion::_validateVersion(StringData version) {
     uassert(40284,
             str::stream() << "featureCompatibilityVersion must be '"
                           << FeatureCompatibilityVersionCommandParser::kVersion40
-                          << "', '"
-                          << FeatureCompatibilityVersionCommandParser::kVersion36
                           << "' or '"
-                          << FeatureCompatibilityVersionCommandParser::kVersion34
+                          << FeatureCompatibilityVersionCommandParser::kVersion36
                           << "'. See "
                           << feature_compatibility_version::kDochubLink
                           << ".",
             version == FeatureCompatibilityVersionCommandParser::kVersion40 ||
-                version == FeatureCompatibilityVersionCommandParser::kVersion36 ||
-                version == FeatureCompatibilityVersionCommandParser::kVersion34);
+                version == FeatureCompatibilityVersionCommandParser::kVersion36);
 }
 
 void FeatureCompatibilityVersion::_runUpdateCommand(OperationContext* opCtx,
@@ -449,27 +386,6 @@ public:
                     FeatureCompatibilityVersion::kVersionField,
                     FeatureCompatibilityVersionCommandParser::kVersion36);
                 return;
-            case ServerGlobalParams::FeatureCompatibility::Version::kUpgradingTo36:
-                featureCompatibilityVersionBuilder.append(
-                    FeatureCompatibilityVersion::kVersionField,
-                    FeatureCompatibilityVersionCommandParser::kVersion34);
-                featureCompatibilityVersionBuilder.append(
-                    FeatureCompatibilityVersion::kTargetVersionField,
-                    FeatureCompatibilityVersionCommandParser::kVersion36);
-                return;
-            case ServerGlobalParams::FeatureCompatibility::Version::kDowngradingTo34:
-                featureCompatibilityVersionBuilder.append(
-                    FeatureCompatibilityVersion::kVersionField,
-                    FeatureCompatibilityVersionCommandParser::kVersion34);
-                featureCompatibilityVersionBuilder.append(
-                    FeatureCompatibilityVersion::kTargetVersionField,
-                    FeatureCompatibilityVersionCommandParser::kVersion34);
-                return;
-            case ServerGlobalParams::FeatureCompatibility::Version::kFullyDowngradedTo34:
-                featureCompatibilityVersionBuilder.append(
-                    FeatureCompatibilityVersion::kVersionField,
-                    FeatureCompatibilityVersionCommandParser::kVersion34);
-                return;
             case ServerGlobalParams::FeatureCompatibility::Version::kUnsetDefault36Behavior:
                 // getVersion() does not return this value.
                 MONGO_UNREACHABLE;
diff --git a/src/mongo/db/commands/feature_compatibility_version.h b/src/mongo/db/commands/feature_compatibility_version.h
index 1c3916565b0..02f8e3df7f6 100644
--- a/src/mongo/db/commands/feature_compatibility_version.h
+++ b/src/mongo/db/commands/feature_compatibility_version.h
@@ -75,12 +75,6 @@ public:
         switch (version) {
             case ServerGlobalParams::FeatureCompatibility::Version::kUnsetDefault36Behavior:
                 return FeatureCompatibilityVersionCommandParser::kVersionUnset;
-            case ServerGlobalParams::FeatureCompatibility::Version::kFullyDowngradedTo34:
-                return FeatureCompatibilityVersionCommandParser::kVersion34;
-            case ServerGlobalParams::FeatureCompatibility::Version::kUpgradingTo36:
-                return FeatureCompatibilityVersionCommandParser::kVersionUpgradingTo36;
-            case ServerGlobalParams::FeatureCompatibility::Version::kDowngradingTo34:
-                return FeatureCompatibilityVersionCommandParser::kVersionDowngradingTo34;
             case ServerGlobalParams::FeatureCompatibility::Version::kFullyDowngradedTo36:
                 return FeatureCompatibilityVersionCommandParser::kVersion36;
             case ServerGlobalParams::FeatureCompatibility::Version::kUpgradingTo40:
@@ -95,20 +89,6 @@ public:
     }
 
     /**
-     * Records intent to perform a 3.4 -> 3.6 upgrade by updating the on-disk feature
-     * compatibility version document to have 'version'=3.4, 'targetVersion'=3.6.
-     * Should be called before schemas are modified.
-     */
-    static void setTargetUpgrade_DEPRECATED(OperationContext* opCtx);
-
-    /**
-     * Records intent to perform a 3.6 -> 3.4 downgrade by updating the on-disk feature
-     * compatibility version document to have 'version'=3.4, 'targetVersion'=3.4.
-     * Should be called before schemas are modified.
-     */
-    static void setTargetDowngrade_DEPRECATED(OperationContext* opCtx);
-
-    /**
      * Records intent to perform a 3.6 -> 4.0 upgrade by updating the on-disk feature
      * compatibility version document to have 'version'=3.6, 'targetVersion'=4.0.
      * Should be called before schemas are modified.
@@ -123,9 +103,9 @@ public:
     static void setTargetDowngrade(OperationContext* opCtx);
 
     /**
-     * Records the completion of a 3.4 <-> 3.6 or 3.6 <-> 4.0 upgrade or downgrade by updating the
-     * on-disk feature compatibility version document to have 'version'=version and unsetting the
-     * 'targetVersion' field.
+     * Records the completion of a 3.6 <-> 4.0 upgrade or downgrade by updating the on-disk feature
+     * compatibility version document to have 'version'=version and unsetting the 'targetVersion'
+     * field.
      * Should be called after schemas are modified.
      */
     static void unsetTargetUpgradeOrDowngrade(OperationContext* opCtx, StringData version);
diff --git a/src/mongo/db/commands/feature_compatibility_version_command_parser.cpp b/src/mongo/db/commands/feature_compatibility_version_command_parser.cpp
index c8dd5dfcecc..49ce4128b8d 100644
--- a/src/mongo/db/commands/feature_compatibility_version_command_parser.cpp
+++ b/src/mongo/db/commands/feature_compatibility_version_command_parser.cpp
@@ -38,11 +38,8 @@
 
 namespace mongo {
 
-constexpr StringData FeatureCompatibilityVersionCommandParser::kVersion34;
 constexpr StringData FeatureCompatibilityVersionCommandParser::kVersion36;
 constexpr StringData FeatureCompatibilityVersionCommandParser::kVersion40;
-constexpr StringData FeatureCompatibilityVersionCommandParser::kVersionUpgradingTo36;
-constexpr StringData FeatureCompatibilityVersionCommandParser::kVersionDowngradingTo34;
 constexpr StringData FeatureCompatibilityVersionCommandParser::kVersionUpgradingTo40;
 constexpr StringData FeatureCompatibilityVersionCommandParser::kVersionDowngradingTo36;
 constexpr StringData FeatureCompatibilityVersionCommandParser::kVersionUnset;
@@ -88,15 +85,12 @@ StatusWith<std::string> FeatureCompatibilityVersionCommandParser::extractVersion
     const std::string version = versionElem.String();
 
     if (version != FeatureCompatibilityVersionCommandParser::kVersion40 &&
-        version != FeatureCompatibilityVersionCommandParser::kVersion36 &&
-        version != FeatureCompatibilityVersionCommandParser::kVersion34) {
+        version != FeatureCompatibilityVersionCommandParser::kVersion36) {
         return {ErrorCodes::BadValue,
                 str::stream() << "Invalid command argument. Expected '"
                               << FeatureCompatibilityVersionCommandParser::kVersion40
-                              << "', '"
-                              << FeatureCompatibilityVersionCommandParser::kVersion36
                               << "' or '"
-                              << FeatureCompatibilityVersionCommandParser::kVersion34
+                              << FeatureCompatibilityVersionCommandParser::kVersion36
                               << "', found "
                               << version
                               << " in: "
diff --git a/src/mongo/db/commands/feature_compatibility_version_command_parser.h b/src/mongo/db/commands/feature_compatibility_version_command_parser.h
index 0f19576765e..284471de123 100644
--- a/src/mongo/db/commands/feature_compatibility_version_command_parser.h
+++ b/src/mongo/db/commands/feature_compatibility_version_command_parser.h
@@ -47,11 +47,8 @@ public:
     /**
      * Known server release versions.
      */
-    static constexpr StringData kVersion34 = "3.4"_sd;
     static constexpr StringData kVersion36 = "3.6"_sd;
     static constexpr StringData kVersion40 = "4.0"_sd;
-    static constexpr StringData kVersionUpgradingTo36 = "upgrading to 3.6"_sd;
-    static constexpr StringData kVersionDowngradingTo34 = "downgrading to 3.4"_sd;
     static constexpr StringData kVersionUpgradingTo40 = "upgrading to 4.0"_sd;
     static constexpr StringData kVersionDowngradingTo36 = "downgrading to 3.6"_sd;
     static constexpr StringData kVersionUnset = "Unset"_sd;
diff --git a/src/mongo/db/commands/mr.cpp b/src/mongo/db/commands/mr.cpp
index 54b0d7efaef..a687ec89a29 100644
--- a/src/mongo/db/commands/mr.cpp
+++ b/src/mongo/db/commands/mr.cpp
@@ -423,8 +423,7 @@ void State::prepTempCollection() {
             CollectionOptions options;
             options.setNoIdIndex();
             options.temp = true;
-            if (enableCollectionUUIDs &&
-                serverGlobalParams.featureCompatibility.isSchemaVersion36()) {
+            if (enableCollectionUUIDs) {
                 options.uuid.emplace(UUID::gen());
             }
             incColl = incCtx.db()->createCollection(_opCtx, _config.incLong.ns(), options);
@@ -510,7 +509,7 @@ void State::prepTempCollection() {
 
         CollectionOptions options = finalOptions;
         options.temp = true;
-        if (enableCollectionUUIDs && serverGlobalParams.featureCompatibility.isSchemaVersion36()) {
+        if (enableCollectionUUIDs) {
             // If a UUID for the final output collection was sent by mongos (i.e., the final output
             // collection is sharded), use the UUID mongos sent when creating the temp collection.
             // When the temp collection is renamed to the final output collection, the UUID will be
@@ -1754,8 +1753,7 @@ public:
 
         Config config(dbname, cmdObj.firstElement().embeddedObjectUserCheck());
 
-        if (cmdObj["finalOutputCollIsSharded"].trueValue() &&
-            serverGlobalParams.featureCompatibility.isSchemaVersion36()) {
+        if (cmdObj["finalOutputCollIsSharded"].trueValue()) {
             uassert(ErrorCodes::InvalidOptions,
                     "This shard has feature compatibility version 3.6, so it expects mongos to "
                     "send the UUID to use for the sharded output collection. Was the mapReduce "
diff --git a/src/mongo/db/commands/set_feature_compatibility_version_command.cpp b/src/mongo/db/commands/set_feature_compatibility_version_command.cpp
index e9b510d4120..bccb159e9ef 100644
--- a/src/mongo/db/commands/set_feature_compatibility_version_command.cpp
+++ b/src/mongo/db/commands/set_feature_compatibility_version_command.cpp
@@ -84,11 +84,11 @@ public:
     std::string help() const override {
         std::stringstream h;
         h << "Set the API version exposed by this node. If set to \""
-          << FeatureCompatibilityVersionCommandParser::kVersion34
-          << "\", then 3.6 features are disabled. If \""
           << FeatureCompatibilityVersionCommandParser::kVersion36
-          << "\", then 3.6 features are enabled, and all nodes in the cluster must be version "
-             "3.6. See "
+          << "\", then 4.0 features are disabled. If \""
+          << FeatureCompatibilityVersionCommandParser::kVersion40
+          << "\", then 4.0 features are enabled, and all nodes in the cluster must be binary "
+             "version 4.0. See "
           << feature_compatibility_version::kDochubLink << ".";
         return h.str();
     }
@@ -139,22 +139,6 @@ public:
 
         if (requestedVersion == FeatureCompatibilityVersionCommandParser::kVersion40) {
             uassert(ErrorCodes::IllegalOperation,
-                    "cannot initiate featureCompatibilityVersion upgrade to 4.0 when in 3.4. "
-                    "Upgrade to featureCompatibility 3.6 before upgrading to 4.0.",
-                    actualVersion !=
-                        ServerGlobalParams::FeatureCompatibility::Version::kFullyDowngradedTo34);
-            uassert(ErrorCodes::IllegalOperation,
-                    "cannot initiate featureCompatibilityVersion upgrade to 4.0 while a previous "
-                    "downgrade to 3.4 has not completed. Finish downgrade to 3.4, upgrade to 3.6, "
-                    "and then 4.0.",
-                    actualVersion !=
-                        ServerGlobalParams::FeatureCompatibility::Version::kDowngradingTo34);
-            uassert(ErrorCodes::IllegalOperation,
-                    "cannot initiate featureCompatibilityVersion upgrade to 4.0 while a previous "
-                    "upgrade to 3.6 has not completed. Finish upgrade to 3.6, then upgrade to 4.0.",
-                    actualVersion !=
-                        ServerGlobalParams::FeatureCompatibility::Version::kUpgradingTo36);
-            uassert(ErrorCodes::IllegalOperation,
                     "cannot initiate featureCompatibilityVersion upgrade to 4.0 while a previous "
                     "featureCompatibilityVersion downgrade to 3.6 has not completed. Finish "
                     "downgrade to 3.6, then upgrade to 4.0.",
@@ -172,7 +156,18 @@ public:
 
             FeatureCompatibilityVersion::setTargetUpgrade(opCtx);
 
-            // If config server, upgrade shards *after* upgrading self.
+            {
+                // Take the global lock in S mode to create a barrier for operations taking the
+                // global IX or X locks. This ensures that either
+                //   - The global IX/X locked operation will start after the FCV change, see the
+                //     upgrading to 4.0 FCV and act accordingly.
+                //   - The global IX/X locked operation began prior to the FCV change, is acting on
+                //     that assumption and will finish before upgrade procedures begin right after
+                //     this.
+                Lock::GlobalLock lk(opCtx, MODE_S, Date_t::max());
+            }
+
+            // Upgrade shards before config finishes its upgrade.
             if (serverGlobalParams.clusterRole == ClusterRole::ConfigServer) {
                 uassertStatusOK(
                     ShardingCatalogManager::get(opCtx)->setFeatureCompatibilityVersionOnShards(
@@ -188,11 +183,6 @@ public:
         } else if (requestedVersion == FeatureCompatibilityVersionCommandParser::kVersion36) {
             uassert(ErrorCodes::IllegalOperation,
                     "cannot initiate setting featureCompatibilityVersion to 3.6 while a previous "
-                    "featureCompatibilityVersion downgrade to 3.4 has not completed.",
-                    actualVersion !=
-                        ServerGlobalParams::FeatureCompatibility::Version::kDowngradingTo34);
-            uassert(ErrorCodes::IllegalOperation,
-                    "cannot initiate setting featureCompatibilityVersion to 3.6 while a previous "
                     "featureCompatibilityVersion upgrade to 4.0 has not completed.",
                     actualVersion !=
                         ServerGlobalParams::FeatureCompatibility::Version::kUpgradingTo40);
@@ -206,140 +196,20 @@ public:
                 return true;
             }
 
-            if (actualVersion ==
-                    ServerGlobalParams::FeatureCompatibility::Version::kFullyUpgradedTo40 ||
-                actualVersion ==
-                    ServerGlobalParams::FeatureCompatibility::Version::kDowngradingTo36) {
-                // Downgrading to 3.6.
-
-                FeatureCompatibilityVersion::setTargetDowngrade(opCtx);
-
-                // If config server, downgrade shards *before* downgrading self
-                if (serverGlobalParams.clusterRole == ClusterRole::ConfigServer) {
-                    uassertStatusOK(
-                        ShardingCatalogManager::get(opCtx)->setFeatureCompatibilityVersionOnShards(
-                            opCtx,
-                            CommandHelpers::appendMajorityWriteConcern(
-                                CommandHelpers::appendPassthroughFields(
-                                    cmdObj,
-                                    BSON(FeatureCompatibilityVersion::kCommandName
-                                         << requestedVersion)))));
-                }
-
-                FeatureCompatibilityVersion::unsetTargetUpgradeOrDowngrade(opCtx, requestedVersion);
-            } else {
-                // Upgrading to 3.6.
-
-                invariant(
-                    actualVersion ==
-                        ServerGlobalParams::FeatureCompatibility::Version::kFullyDowngradedTo34 ||
-                    actualVersion ==
-                        ServerGlobalParams::FeatureCompatibility::Version::kUpgradingTo36);
-
-                FeatureCompatibilityVersion::setTargetUpgrade_DEPRECATED(opCtx);
-
-                {
-                    // Take the global lock in S mode to create a barrier for createCollection
-                    // execution, which takes a database exclusive lock. This ensures that either
-                    //   - createCollection will start after the FCV change and see the upgrading to
-                    //     3.6 FCV and create the collection with a UUID.
-                    //   - createCollection that began prior to the FCV change, and is creating a
-                    //     collection without UUID, will finish before the updateUUIDSchemaVersion
-                    //     call below fetches a list of collections to which to add UUIDs for any
-                    //     without a UUID.
-                    // Otherwise createCollection may determine not to add a UUID before the FCV
-                    // change, but then actually create the collection after the update below
-                    // identifies all of the databases to update with UUIDs.
-                    Lock::GlobalLock lk(opCtx, MODE_S, Date_t::max());
-                }
-
-                // First put UUIDs in the storage layer metadata. UUIDs will be generated for
-                // unsharded collections; shards will query the config server for sharded collection
-                // UUIDs. Remove after 3.4 -> 3.6 upgrade.
-                updateUUIDSchemaVersion(opCtx, /*upgrade*/ true);
-
-                // If config server, upgrade shards *after* upgrading self.
-                if (serverGlobalParams.clusterRole == ClusterRole::ConfigServer) {
-                    // Remove after 3.4 -> 3.6 upgrade.
-                    ShardingCatalogManager::get(opCtx)->generateUUIDsForExistingShardedCollections(
-                        opCtx);
-
-                    uassertStatusOK(
-                        ShardingCatalogManager::get(opCtx)->setFeatureCompatibilityVersionOnShards(
-                            opCtx,
-                            CommandHelpers::appendMajorityWriteConcern(
-                                CommandHelpers::appendPassthroughFields(
-                                    cmdObj,
-                                    BSON(FeatureCompatibilityVersion::kCommandName
-                                         << requestedVersion)))));
-                }
-
-                // Fail after adding UUIDs but before updating the FCV document.
-                if (MONGO_FAIL_POINT(featureCompatibilityUpgrade)) {
-                    exitCleanly(EXIT_CLEAN);
-                }
-
-                FeatureCompatibilityVersion::unsetTargetUpgradeOrDowngrade(opCtx, requestedVersion);
-            }
-        } else {
-            invariant(requestedVersion == FeatureCompatibilityVersionCommandParser::kVersion34);
-
-            uassert(ErrorCodes::IllegalOperation,
-                    "cannot initiate featureCompatibilityVersion downgrade to 3.4 when in 4.0. "
-                    "Downgrade to featureCompatibility 3.6 before downgrading to 3.4",
-                    actualVersion !=
-                        ServerGlobalParams::FeatureCompatibility::Version::kFullyUpgradedTo40);
-            uassert(ErrorCodes::IllegalOperation,
-                    "cannot initiate featureCompatibilityVersion downgrade to 3.4 while a previous "
-                    "upgrade to 4.0 has not completed. Finish upgrade to 4.0, downgrade to 3.6, "
-                    "and then 3.4.",
-                    actualVersion !=
-                        ServerGlobalParams::FeatureCompatibility::Version::kUpgradingTo40);
-            uassert(ErrorCodes::IllegalOperation,
-                    "cannot initiate featureCompatibilityVersion downgrade to 3.4 while a previous "
-                    "downgrade to 3.6 has not completed. Finish downgrade to 3.6, then downgrade "
-                    " to 3.4.",
-                    actualVersion !=
-                        ServerGlobalParams::FeatureCompatibility::Version::kDowngradingTo36);
-            uassert(ErrorCodes::IllegalOperation,
-                    "cannot initiate featureCompatibilityVersion downgrade to 3.4 while a previous "
-                    "featureCompatibilityVersion upgrade to 3.6 has not completed. Finish upgrade "
-                    "to 3.6, then downgrade to 3.4.",
-                    actualVersion !=
-                        ServerGlobalParams::FeatureCompatibility::Version::kUpgradingTo36);
-
-            if (actualVersion ==
-                ServerGlobalParams::FeatureCompatibility::Version::kFullyDowngradedTo34) {
-                // Set the client's last opTime to the system last opTime so no-ops wait for
-                // writeConcern.
-                repl::ReplClientInfo::forClient(opCtx->getClient())
-                    .setLastOpToSystemLastOpTime(opCtx);
-                return true;
-            }
-
-            FeatureCompatibilityVersion::setTargetDowngrade_DEPRECATED(opCtx);
+            FeatureCompatibilityVersion::setTargetDowngrade(opCtx);
 
             {
-                // Take the global lock in S mode to create a barrier for createCollection
-                // execution, which takea a database exclusive lock. This ensures that either
-                //   - createCollection will start after the FCV change, see downgrading to 3.4 FCV
-                //     and not create the collection with a UUID.
-                //   - createCollection that began prior to the FCV change, and is creating a
-                //     collection with UUID, will finish before the updateUUIDSchemaVersion call
-                //     below fetches a list of collections from which to remove UUIDs for any with
-                //     a UUID.
-                // Otherwise createCollection may determine to add a UUID before the FCV change, but
-                // then actually create the collection after the update below identifies all of the
-                // databases from which to remove UUIDs.
+                // Take the global lock in S mode to create a barrier for operations taking the
+                // global IX or X locks. This ensures that either
+                //   - The global IX/X locked operation will start after the FCV change, see the
+                //     downgrading to 3.6 FCV and act accordingly.
+                //   - The global IX/X locked operation began prior to the FCV change, is acting on
+                //     that assumption and will finish before downgrade procedures begin right after
+                //     this.
                 Lock::GlobalLock lk(opCtx, MODE_S, Date_t::max());
             }
 
-            // Fail after updating the FCV document but before removing UUIDs.
-            if (MONGO_FAIL_POINT(featureCompatibilityDowngrade)) {
-                exitCleanly(EXIT_CLEAN);
-            }
-
-            // If config server, downgrade shards *before* downgrading self.
+            // Downgrade shards before config finishes its downgrade.
             if (serverGlobalParams.clusterRole == ClusterRole::ConfigServer) {
                 uassertStatusOK(
                     ShardingCatalogManager::get(opCtx)->setFeatureCompatibilityVersionOnShards(
@@ -351,9 +221,6 @@ public:
                                      << requestedVersion)))));
             }
 
-            // Remove after 3.6 -> 3.4 downgrade.
-            updateUUIDSchemaVersion(opCtx, /*upgrade*/ false);
-
             FeatureCompatibilityVersion::unsetTargetUpgradeOrDowngrade(opCtx, requestedVersion);
         }
 
diff --git a/src/mongo/db/commands/validate.cpp b/src/mongo/db/commands/validate.cpp
index eb7a3175ee9..b6fd8a4b9e2 100644
--- a/src/mongo/db/commands/validate.cpp
+++ b/src/mongo/db/commands/validate.cpp
@@ -207,18 +207,11 @@ public:
         bool skipUUIDCheck = nss.coll() == "system.indexes" || nss.coll() == "system.namespaces";
 
         if (!skipUUIDCheck) {
-            ServerGlobalParams::FeatureCompatibility::Version version =
-                serverGlobalParams.featureCompatibility.getVersion();
-
-            if (version >=
-                ServerGlobalParams::FeatureCompatibility::Version::kFullyDowngradedTo36) {
-                // All collections must have a UUID.
-                if (!opts.uuid) {
-                    results.errors.push_back(str::stream() << "UUID missing on collection "
-                                                           << nss.ns()
-                                                           << " but SchemaVersion=3.6");
-                    results.valid = false;
-                }
+            // All collections must have a UUID.
+            if (!opts.uuid) {
+                results.errors.push_back(str::stream() << "UUID missing on collection " << nss.ns()
+                                                       << " but SchemaVersion=3.6");
+                results.valid = false;
             }
         }
 
diff --git a/src/mongo/db/repair_database_and_check_version.cpp b/src/mongo/db/repair_database_and_check_version.cpp
index bf847c31d1a..0b627043dce 100644
--- a/src/mongo/db/repair_database_and_check_version.cpp
+++ b/src/mongo/db/repair_database_and_check_version.cpp
@@ -395,25 +395,7 @@ StatusWith<bool> repairDatabasesAndCheckVersion(OperationContext* opCtx) {
                     // On startup, if the version is in an upgrading or downrading state, print a
                     // warning.
                     if (version ==
-                        ServerGlobalParams::FeatureCompatibility::Version::kUpgradingTo36) {
-                        log() << "** WARNING: A featureCompatibilityVersion upgrade did not "
-                              << "complete." << startupWarningsLog;
-                        log() << "**          The current featureCompatibilityVersion is "
-                              << FeatureCompatibilityVersion::toString(version) << "."
-                              << startupWarningsLog;
-                        log() << "**          To fix this, use the setFeatureCompatibilityVersion "
-                              << "command to resume upgrade to 3.6." << startupWarningsLog;
-                    } else if (version == ServerGlobalParams::FeatureCompatibility::Version::
-                                              kDowngradingTo34) {
-                        log() << "** WARNING: A featureCompatibilityVersion downgrade did not "
-                              << "complete. " << startupWarningsLog;
-                        log() << "**          The current featureCompatibilityVersion is "
-                              << FeatureCompatibilityVersion::toString(version) << "."
-                              << startupWarningsLog;
-                        log() << "**          To fix this, use the setFeatureCompatibilityVersion "
-                              << "command to resume downgrade to 3.4." << startupWarningsLog;
-                    } else if (version ==
-                               ServerGlobalParams::FeatureCompatibility::Version::kUpgradingTo40) {
+                        ServerGlobalParams::FeatureCompatibility::Version::kUpgradingTo40) {
                         log() << "** WARNING: A featureCompatibilityVersion upgrade did not "
                               << "complete. " << startupWarningsLog;
                         log() << "**          The current featureCompatibilityVersion is "
diff --git a/src/mongo/db/repl/check_quorum_for_config_change.cpp b/src/mongo/db/repl/check_quorum_for_config_change.cpp
index e1a2321d91e..52f7d3ce361 100644
--- a/src/mongo/db/repl/check_quorum_for_config_change.cpp
+++ b/src/mongo/db/repl/check_quorum_for_config_change.cpp
@@ -90,10 +90,7 @@ std::vector<RemoteCommandRequest> QuorumChecker::getRequests() const {
         hbArgs.setSetName(_rsConfig->getReplSetName());
         hbArgs.setProtocolVersion(1);
         hbArgs.setConfigVersion(_rsConfig->getConfigVersion());
-        if (serverGlobalParams.featureCompatibility.getVersion() !=
-            ServerGlobalParams::FeatureCompatibility::Version::kFullyDowngradedTo34) {
-            hbArgs.setHeartbeatVersion(1);
-        }
+        hbArgs.setHeartbeatVersion(1);
         hbArgs.setCheckEmpty(isInitialConfig);
         hbArgs.setSenderHost(myConfig.getHostAndPort());
         hbArgs.setSenderId(myConfig.getId());
@@ -103,10 +100,7 @@ std::vector<RemoteCommandRequest> QuorumChecker::getRequests() const {
         ReplSetHeartbeatArgsV1 hbArgs;
         hbArgs.setSetName(_rsConfig->getReplSetName());
         hbArgs.setConfigVersion(_rsConfig->getConfigVersion());
-        if (serverGlobalParams.featureCompatibility.getVersion() !=
-            ServerGlobalParams::FeatureCompatibility::Version::kFullyDowngradedTo34) {
-            hbArgs.setHeartbeatVersion(1);
-        }
+        hbArgs.setHeartbeatVersion(1);
         if (isInitialConfig) {
             hbArgs.setCheckEmpty();
         }
diff --git a/src/mongo/db/repl/topology_coordinator.cpp b/src/mongo/db/repl/topology_coordinator.cpp
index e8b3710f283..686f7008dd3 100644
--- a/src/mongo/db/repl/topology_coordinator.cpp
+++ b/src/mongo/db/repl/topology_coordinator.cpp
@@ -940,10 +940,7 @@ std::pair<ReplSetHeartbeatArgs, Milliseconds> TopologyCoordinator::prepareHeartb
         hbArgs.setSetName(ourSetName);
         hbArgs.setConfigVersion(-2);
     }
-    if (serverGlobalParams.featureCompatibility.getVersion() !=
-        ServerGlobalParams::FeatureCompatibility::Version::kFullyDowngradedTo34) {
-        hbArgs.setHeartbeatVersion(1);
-    }
+    hbArgs.setHeartbeatVersion(1);
 
     const Milliseconds timeoutPeriod(
         _rsConfig.isInitialized() ? _rsConfig.getHeartbeatTimeoutPeriodMillis()
@@ -979,10 +976,7 @@ std::pair<ReplSetHeartbeatArgsV1, Milliseconds> TopologyCoordinator::prepareHear
         hbArgs.setConfigVersion(-2);
         hbArgs.setTerm(OpTime::kInitialTerm);
     }
-    if (serverGlobalParams.featureCompatibility.getVersion() !=
-        ServerGlobalParams::FeatureCompatibility::Version::kFullyDowngradedTo34) {
-        hbArgs.setHeartbeatVersion(1);
-    }
+    hbArgs.setHeartbeatVersion(1);
 
     const Milliseconds timeoutPeriod(
         _rsConfig.isInitialized() ? _rsConfig.getHeartbeatTimeoutPeriodMillis()
diff --git a/src/mongo/db/s/config/configsvr_shard_collection_command.cpp b/src/mongo/db/s/config/configsvr_shard_collection_command.cpp
index 1ad8459f3f1..4dc2b459384 100644
--- a/src/mongo/db/s/config/configsvr_shard_collection_command.cpp
+++ b/src/mongo/db/s/config/configsvr_shard_collection_command.cpp
@@ -643,11 +643,6 @@ void migrateAndFurtherSplitInitialChunks(OperationContext* opCtx,
 
 boost::optional<UUID> getUUIDFromPrimaryShard(const NamespaceString& nss,
                                               ScopedDbConnection& conn) {
-    // UUIDs were introduced in featureCompatibilityVersion 3.6.
-    if (!serverGlobalParams.featureCompatibility.isSchemaVersion36()) {
-        return boost::none;
-    }
-
     // Obtain the collection's UUID from the primary shard's listCollections response.
     BSONObj res;
     {
diff --git a/src/mongo/db/s/config/sharding_catalog_manager_shard_operations.cpp b/src/mongo/db/s/config/sharding_catalog_manager_shard_operations.cpp
index 79489af4c9e..5d93e1d7a62 100644
--- a/src/mongo/db/s/config/sharding_catalog_manager_shard_operations.cpp
+++ b/src/mongo/db/s/config/sharding_catalog_manager_shard_operations.cpp
@@ -340,20 +340,14 @@ StatusWith<ShardType> ShardingCatalogManager::_validateHostAsShard(
     }
     if (serverGlobalParams.featureCompatibility.getVersion() >
         ServerGlobalParams::FeatureCompatibility::Version::kFullyDowngradedTo36) {
-        // If FCV 4.0, or upgrading to / downgrading from, wire version must be LATEST.
+        // If the cluster's FCV is 4.0, or upgrading to / downgrading from, the node being added
+        // must be a v4.0 binary.
         invariant(maxWireVersion == WireVersion::LATEST_WIRE_VERSION);
-    } else if (serverGlobalParams.featureCompatibility.getVersion() >
-                   ServerGlobalParams::FeatureCompatibility::Version::kFullyDowngradedTo34 &&
-               serverGlobalParams.featureCompatibility.getVersion() <=
-                   ServerGlobalParams::FeatureCompatibility::Version::kFullyDowngradedTo36) {
-        // If FCV 3.6, or upgrading to / downgrading from, wire version must be v3.6
-        // LATEST_WIRE_VERSION or greater.
-        invariant(maxWireVersion >= WireVersion::LATEST_WIRE_VERSION - 1);
     } else {
-        // If FCV 3.4, wire version cannot be less than v3.4 LATEST_WIRE_VERSION.
+        // If the cluster's FCV is 3.6, the node being added must be a v3.6 or v4.0 binary.
         invariant(serverGlobalParams.featureCompatibility.getVersion() ==
-                  ServerGlobalParams::FeatureCompatibility::Version::kFullyDowngradedTo34);
-        invariant(maxWireVersion >= WireVersion::LATEST_WIRE_VERSION - 2);
+                  ServerGlobalParams::FeatureCompatibility::Version::kFullyDowngradedTo36);
+        invariant(maxWireVersion >= WireVersion::LATEST_WIRE_VERSION - 1);
     }
 
     // Check whether there is a master. If there isn't, the replica set may not have been
diff --git a/src/mongo/db/s/migration_destination_manager.cpp b/src/mongo/db/s/migration_destination_manager.cpp
index a29bbc368bf..ac2746f180a 100644
--- a/src/mongo/db/s/migration_destination_manager.cpp
+++ b/src/mongo/db/s/migration_destination_manager.cpp
@@ -561,25 +561,21 @@ void MigrationDestinationManager::_migrateDriver(OperationContext* opCtx,
             donorOptionsBob.appendElements(entry["options"].Obj());
         }
 
-        if (serverGlobalParams.featureCompatibility.isSchemaVersion36()) {
-            BSONObj info;
-            if (entry["info"].isABSONObj()) {
-                info = entry["info"].Obj();
-            }
-            if (info["uuid"].eoo()) {
-                setStateFailWarn(str::stream()
-                                 << "The donor shard did not return a UUID for collection "
-                                 << _nss.ns()
-                                 << " as part of its listCollections response: "
-                                 << entry
-                                 << ", but this node expects to see a UUID since its "
-                                    "feature compatibility version is 3.6. Please follow "
-                                    "the online documentation to set the same feature "
-                                    "compatibility version across the cluster.");
-                return;
-            }
-            donorOptionsBob.append(info["uuid"]);
+        BSONObj info;
+        if (entry["info"].isABSONObj()) {
+            info = entry["info"].Obj();
+        }
+        if (info["uuid"].eoo()) {
+            setStateFailWarn(str::stream()
+                             << "The donor shard did not return a UUID for collection "
+                             << _nss.ns()
+                             << " as part of its listCollections response: "
+                             << entry
+                             << ", but this node expects to see a UUID.");
+            return;
         }
+        donorOptionsBob.append(info["uuid"]);
+
         donorOptions = donorOptionsBob.obj();
     }
 
diff --git a/src/mongo/db/server_options.h b/src/mongo/db/server_options.h
index e0a34ddb39c..d5c2e141826 100644
--- a/src/mongo/db/server_options.h
+++ b/src/mongo/db/server_options.h
@@ -152,25 +152,25 @@ struct ServerGlobalParams {
          * Features can be gated for specific versions, or ranges of versions above or below some
          * minimum or maximum version, respectively.
          *
-         * The legal enum (and featureCompatiblityVersion document) states are:
-         *
-         * kFullyDowngradedTo34
-         * (3.4, Unset): Only 3.4 features are available, and new and existing storage
-         *               engine entries use the 3.4 format
-         *
-         * kUpgradingTo36
-         * (3.4, 3.6): Only 3.4 features are available, but new storage engine entries
-         *             use the 3.6 format, and existing entries may have either the
-         *             3.4 or 3.6 format
+         * The legal enum (and featureCompatibilityVersion document) states are:
          *
          * kFullyDowngradedTo36
-         * (3.6, Unset): 3.6 features are available, and new and existing storage
+         * (3.6, Unset): Only 3.6 features are available, and new and existing storage
          *               engine entries use the 3.6 format
          *
-         * kDowngradingTo34
-         * (3.4, 3.4): Only 3.4 features are available and new storage engine
-         *             entries use the 3.4 format, but existing entries may have
-         *             either the 3.4 or 3.6 format
+         * kUpgradingTo40
+         * (3.6, 4.0): Only 3.6 features are available, but new storage engine entries
+         *             use the 4.0 format, and existing entries may have either the
+         *             3.6 or 4.0 format
+         *
+         * kFullyUpgradedTo40
+         * (4.0, Unset): 4.0 features are available, and new and existing storage
+         *               engine entries use the 4.0 format
+         *
+         * kDowngradingTo36
+         * (3.6, 3.6): Only 3.6 features are available and new storage engine
+         *             entries use the 3.6 format, but existing entries may have
+         *             either the 3.6 or 4.0 format
          *
          * kUnsetDefault36Behavior
          * (Unset, Unset): This is the case on startup before the fCV document is
@@ -178,20 +178,16 @@ struct ServerGlobalParams {
          *                 false, and getVersion() will return the default
          *                 (kFullyDowngradedTo36).
          *
-         * TODO: update this comment to 3.6/4.0 when FCV 3.4 is removed (SERVER-33179).
          */
         enum class Version {
             // The order of these enums matter, higher upgrades having higher values, so that
             // features can be active or inactive if the version is higher than some minimum or
             // lower than some maximum, respectively.
             kUnsetDefault36Behavior = 0,
-            kFullyDowngradedTo34 = 1,
-            kDowngradingTo34 = 2,
-            kUpgradingTo36 = 3,
-            kFullyDowngradedTo36 = 4,
-            kDowngradingTo36 = 5,
-            kUpgradingTo40 = 6,
-            kFullyUpgradedTo40 = 7,
+            kFullyDowngradedTo36 = 1,
+            kDowngradingTo36 = 2,
+            kUpgradingTo40 = 3,
+            kFullyUpgradedTo40 = 4,
         };
 
         /**
@@ -219,10 +215,9 @@ struct ServerGlobalParams {
             return _version.store(version);
         }
 
-        // This determines whether to give Collections UUIDs upon creation.
-        const bool isSchemaVersion36() {
-            return (getVersion() >= Version::kFullyDowngradedTo36 ||
-                    getVersion() == Version::kUpgradingTo36);
+        const bool isVersionUpgradingOrUpgraded() {
+            return (getVersion() == Version::kUpgradingTo40 ||
+                    getVersion() == Version::kFullyUpgradedTo40);
         }
 
     private:
diff --git a/src/mongo/db/storage/wiredtiger/wiredtiger_kv_engine.cpp b/src/mongo/db/storage/wiredtiger/wiredtiger_kv_engine.cpp
index cbd613857a8..016ba8a7a61 100644
--- a/src/mongo/db/storage/wiredtiger/wiredtiger_kv_engine.cpp
+++ b/src/mongo/db/storage/wiredtiger/wiredtiger_kv_engine.cpp
@@ -510,21 +510,21 @@ void WiredTigerKVEngine::cleanShutdown() {
         // There are two cases to consider where the server will shutdown before the in-memory FCV
         // state is set. One is when `EncryptionHooks::restartRequired` is true. The other is when
         // the server shuts down because it refuses to acknowledge an FCV value more than one
-        // version behind (e.g: 3.6 errors when reading 3.2).
+        // version behind (e.g: 4.0 errors when reading 3.4).
         //
         // In the first case, we ideally do not perform a file format downgrade (but it is
-        // acceptable). In the second, the server must downgrade to allow a 3.4 binary to start
+        // acceptable). In the second, the server must downgrade to allow a 3.6 binary to start
         // up. Ideally, our internal FCV value would allow for older values, even if only to
         // immediately shutdown. This would allow downstream logic, such as this method, to make
         // an informed decision.
         const bool needsDowngrade = !_readOnly &&
             serverGlobalParams.featureCompatibility.getVersion() ==
-                ServerGlobalParams::FeatureCompatibility::Version::kFullyDowngradedTo34;
+                ServerGlobalParams::FeatureCompatibility::Version::kFullyDowngradedTo36;
 
         invariantWTOK(_conn->close(_conn, closeConfig));
         _conn = nullptr;
 
-        // If FCV 3.4, enable WT logging on all tables.
+        // If FCV 3.6, enable WT logging on all tables.
         if (needsDowngrade) {
             // Steps for downgrading:
             //
@@ -534,8 +534,8 @@ void WiredTigerKVEngine::cleanShutdown() {
             // 2) Enable WiredTiger logging on all tables.
             //
             // 3) Reconfigure the WiredTiger to release compatibility 2.9. The WiredTiger version
-            //    shipped with MongoDB 3.4 will always refuse to start up without this reconfigure
-            //    being successful. Doing this last prevents MongoDB running in 3.4 with only some
+            //    shipped with MongoDB 3.6 will always refuse to start up without this reconfigure
+            //    being successful. Doing this last prevents MongoDB running in 3.6 with only some
             //    underlying tables being logged.
             LOG(1) << "Downgrading WiredTiger tables to release compatibility 2.9";
             WT_CONNECTION* conn;
diff --git a/src/mongo/s/commands/cluster_set_feature_compatibility_version_cmd.cpp b/src/mongo/s/commands/cluster_set_feature_compatibility_version_cmd.cpp
index ff753df8c69..21f2a9b259e 100644
--- a/src/mongo/s/commands/cluster_set_feature_compatibility_version_cmd.cpp
+++ b/src/mongo/s/commands/cluster_set_feature_compatibility_version_cmd.cpp
@@ -67,15 +67,13 @@ public:
     }
 
     std::string help() const override {
-        // TODO: update this comment when FCV 3.4 is removed (SERVER-33179).
         return str::stream()
             << "Set the API version for the cluster. If set to \""
-            << FeatureCompatibilityVersionCommandParser::kVersion34
-            << "\", then 3.6 features are disabled. If \""
             << FeatureCompatibilityVersionCommandParser::kVersion36
-            << "\", then 3.6 features are enabled, and all nodes in the cluster must be version "
-            << "3.6. If \"4.0\", then 4.0 features are enabled. See "
-            << feature_compatibility_version::kDochubLink << ".";
+            << "\", then 4.0 features are disabled. If \""
+            << FeatureCompatibilityVersionCommandParser::kVersion40
+            << "\", then 4.0 features are enabled, and all nodes in the cluster must be version "
+            << "4.0. See " << feature_compatibility_version::kDochubLink << ".";
     }
 
     Status checkAuthForCommand(Client* client,