In Pass Monitor it shows which logins don't have 2FA enabled yet, but do support it. Pass Monitor does, however, not show a list of login's that support the use of a security key. This could be helpful in further enhancing the security of accounts. Now it has to be checked manually.

Maybe it does not have to be a 'missing security key' feature, as the key is not stored in Proton Pass, but just a check to see which login's support it. Or a way to check the items yourself if a key is added.