Many people have shared some issues with allowing non-alphanumeric characters, but we already have some users on Discord with non-alphanumeric nicknames, so I don't get it why it would be a problem for it to be added into the Minecraft server.
The issue is here people would be abusing the allowunsafe permission node to use the obfuscation decoration with chinese characters which can really lag a game. I know you can use color codes and such in your Discord names but that's never been supported; it's actually an issue with stripColor that up until now nobody has bothered to fix. It was fixed a few months ago but with the release cycle being what it is I'd wager it'll be a while until the fix version gets deployed.
Additionally we already can use non-alphanumeric tags already, which is even more of a reason why we should also be allowed to use non-alphanumeric nicknames.
The obfuscation decoration is stripped from tags, and tags aren't as integral as a display name.
Vouch on the condition /nickclean removes these characters and anyone that abuses non-alphanumeric characters is punished as appropriate.
This is just extra effort for the admins which they wouldn't have to put in if the node was disabled.