I'm really surprised someone technically minded thought it's a good idea to not allow non ASCII alphanumerics in a username.
Unicode has been a thing since 1988. Names have included non a-z characters since forever.
Unicode has been a thing since 1988. Names have included non a-z characters since forever.
String come up again and again as a hard issue to deal with especially once your start looking at Unicode. I think it would be very reasonable to assume only ASCII works and even then it doesn't always work!
Names have been spoken and hand-written since forever yet somehow computers aren't good at that so we all tolerate converting them to printed-looking text. Nobody cares, it doesn't matter.