Good day to you. I recently came across this very same issue and attempted the PHP snippet that you provided but did not have any luck. Upon closer reading, however, I see that you also mention to try and set the plugin as a MU-Plugin, which isn’t an option for me due to maintenance and update agreements on my client end. Should the above snippet still work, or, might you suggest something better so that I can turn this UI off within the WP backend? I’d like to keep this option away from my clients so that they do not get confused at both New User Registration and Edit User scenarios.

Thank you.