The first revision Yubikey 5’s support resident keys, just not the credential management API, at least that is my understanding.
I had a Yubikey 5 with the first firmware revision and resident keys worked fine.
I had a Yubikey 5 with the first firmware revision and resident keys worked fine.
Because the first firmware revision lacks the credential management API, there's no way to list the resident keys on the device or to delete individual keys. The only way to delete resident keys AFAIK is to reset the FIDO2 application on the key.