Generally properly written code should be able to handle any string, with the scariest cases being single quote (( ' )) if someone was using SQL to store data without either using the proper sanitization to escape input data, or using prepared statements.

Beyond that care would have to be taken to escape any special characters where usernames are transported (I'm not sure what the restrictions the IRC protocol enforces), and formats where someone might be writing usernames to disk with the intention to load back later.

Basically in code you can always use escape sequences to represent characters in a string that would otherwise be unsafe though, so that should not be an issue.

Some special symbols might hold certain functions in code and preventing these from being used ensures that usernames are more 'stable'.

/ | . * % are symbols that have certain functions and I can understand them not being allowed.

- + & I do think should be allowed. & does have some functions, but that's usually in combination with # or ; which isn't allowed anyway.

In addition to this & the other reply, the name I use in games is Jōtarōゴゴゴ, it would be welcomed and appreciated if we were able to use characters from other languages in our usernames as well as macrons on top of letters (ā, ē, ī. ō, ū).

I would like to use special characters in Display Name.

My current display name is ゆうた敗北 and I want to change it to ゆうた(敗北) or ゆうた「敗北」.
Please allow to use parentheses and Japanese brackets.

Note that Display Name is different from Username:
https://blog.twitch.tv/en/2016/08/22/localized-display-names-e00ee8d3250a/

Having the feature to be able to use frequently used special characters like * & + - for example in usernames. Thank you in advance.

I found underscore works but I personally wanted to include a space or a colon but cannot.