In Compass Indexes UI, It is not possible to tell which indexes are the shard key indexes. A user may accidentally drop a shard key index. This leaves you without an index on the shard key, which can have big performance implications. A customer on 4.4 (OmniPod) just hit this issue.

It would be good to clearly indicate with indexes are indexes on the shard key fields to prevent customers from making a disastrous mistake.