That’s great if every external library and application also expects usernames that can contain every character, but mostly they won’t. If you’re very good about testing, you might catch every instance where they don’t, but that doesn’t mean you can necessarily fix them / get them fixed.

Plus, there’s just displaying them. If you have sane limits on usernames, you can have sane user management tools that do things that list users vs. storage used, or whatever. If a username might contain backspace characters or who knows what else, all those tools have to be made more complicated too.

In addition, there’s user support. If you allow usernames to contain zalgo text, you’re going to have far more users needing support because they’re having trouble entering their username. You’re also going to give your support people nightmares trying to help those users out.

“Ok, I see you entered your username as “s̶u̵g̶a̴r̴_̵i̷n̴_̵y̶o̸u̷r̸-̴t̴e̷a̴” not “s̶u̵g̵a̶r̷_̶i̴n̸_̸y̵o̴u̵r̷_̸t̵e̵a̷”, could you try re-entering that username and see if that fixes the problem?”