Most of Unicode characters are allowed, like non-latin alphabets (japanese, arabic, cyrillic, chinese ...), special accentuations on character or even most of the emojis ๐. And they can be mixed with latin character as well, so
i๐ฅฐpasta would be a valid subdomain to choose. Everything that can be encoded as Punycode is available, see here for more information :
Punycode Wikipedia