Any attempt to downgrade a sharded cluster from FCV 8.0 to 7.0 fails if there are any tracked collections (collections where data and primary shard reside on different shards). In this circumstance, the current error message displayed by the setFeatureCompatibilityVersion command is:

MongoServerError[CannotDowngrade]: OperationFailed: In order to untrack the collection it must be located in the primary db

This error message is unrelated to the downgrade operation itself and fails to provide important information, such as notifying users that they need to upgrade the FCV back to 8.0 before issuing any moveCollection commands. A more helpful error message would be:

The downgrade process failed due to the presence of tracked collections. Please upgrade the FCV back to 8.0 before moving these collections to their primary shards.