Of the languages I've checked, Perl, Python, and JavaScript (in Chrome and Firefox) do not. PHP does.
> invalid character U+2081 '₁' in identifier
Though that's also not surprising given how poor overall Unicode support is
But AutoHotkey does allow it
super² := 1, sub₂ := 2
Of the languages I've checked, Perl, Python, and JavaScript (in Chrome and Firefox) do not. PHP does.