I would like to suggest a feature that allows me to keep information which security key (yubikey, nitrokey and the like) I have and for which accounts I use them. This is especially important for those service where a username is still used/preferred for whatever reason. I could think of a page where I can add my security keys as reference data, e.g. by just giving it a name and in every (fido2-enabled service) I get a checkbox for every key and can check them or not. By clicking/tapping a key in the reference data I get a quick overview on which accounts I use it.
Motivation: I would like to be able to quickly unregister lost keys without the need to remember where I registered it earlier.
Some solve this by adding an emoji into the name field (not the username) for each vault entry. Others add a keyword to the description field and then use advanced search to find matching entries.
Personally, I create a custom field named Yubikeys, in which the value is a whitespace separated list of key names (e.g., YKblue_02 YKbio_01 YK5_01). This allows me to search using >fields:Yubikeys to find all entries with associated Yubikeys, or >fields:YK5_01 to find all entries associated with the key that was named YK5_01.
In addition, I create a dedicated vault item for each hardware key, specifying the name given to the key, and including a list of all login items that use the key.
I agree that it would be a useful feature to have some such functionality built in to the Bitwarden apps.