The identity name is still used in some datastructures. E.g. the user home or in qti editor files. All those references should be refactored and migrated using an updater code. After that, the identity name should not be used for anything anymore.
An interesting ideal is also to use the identity name to give the identity a globally unique identifier which help reducing conflicts when importing / exporting users. But in any case, the username should not be used for login process, this should use the name stored in the login provider credentials