IMO the reason most people don't know that there are official guidelines on what #Unicode codepoint sequences constitute a valid identifier is because languages largely don't bother to even discover that the standard exists, let alone implement it.

#Python is an exception to the rule, it has had UAX#31 support since Python 3.0¹²

C++ has switched over to this standard as of C++23 although I do not know all of the details. Fun fact: gcc and Clang are both perfectly happy to let you use a zero-width space in an identifier in earlier versions of C++.

¹ https://docs.python.org/3.0/reference/lexical_analysis.html#identifiers-and-keywords
² see PEP 3131 for historical details: https://peps.python.org/pep-3131/