A check in shardsvr_drop_collection_command prevents a user from dropping a timeseries using the "bucket" namespace for a tracked collection. After SERVER-83878 the constraint  has been relaxed and in case the registered collection is "unsplittable" the drop is possible. That was done to ensure feature parity with 7.0 where dropping a timeseries from it's bucket nss was possible for an unsharded collection.

However, as part of SERVER-83878 it has been noticed it is possible to remove completely that check even for sharded collection as the collection will be removed successfully. 

In general, that check is not safe as performed outside the ddl lock. Any shardCollection might occur after the check, causing the drop to remove the sharded collection.