At the Shard Key Indexes documentation it is written: "If you drop the last valid index for the shard key, recover by recreating an index on just the shard key". It is not valid anymore.
In addition, we should specify that it is not possible to drop or hide the last valid index for the shard key.

Description of Linked Ticket

A hidden index helps users evaluate the potential impact of dropping an index without actually dropping the index. That way, the user can just unhide the index if the impact is bad. Hidden and dropped indexes should therefore have the same behavior.

For sharding, however, we allow a shard key index that cannot be dropped to be hidden. In order to maintain consistency, we should not allow users to hide a shard key index that cannot be dropped.