If I define a variable as "groß", does "GROSS" or "GROẞ" match it (or both, which probably implies "gross" would match as well)? What about "ê" and "E"? Or the infamous i/I/İ/ı debacle, which could make matching "insane" to "INSANE" locale-dependent? How do you define case-insensitivity in a way that makes sense?
See Normalization Form KC and Clause 21 of ISO/IEC 10646:2017.
"Normalization forms are the mechanisms allowing the selection of a unique coded representation among alternative; but equivalent coded text representations of the same text. Normalization forms for use with ISO/IEC 10646 are specified in the Unicode Standard UAX#15..." yada yada
Also Unicode identifiers aren't rare in terms of language support. Most of the popular languages support them--C/C++, C#, Java, PHP, Python, Perl, Swift, Go, Rust, Ruby, JavaScript, even Ada. It's actually difficult to find a popular language that prohibits Unicode identifiers entirely (MATLAB does, not sure about Visual Basic).