Steven D'Aprano writes:
If I name a variable "x2", what is the "one simple or obvious interpretation" that such an identifier presumably has? If standard, ASCII-only identifiers don't have a single interpretation, why should identifiers like σ² be held to that requirement?
Because subscripts and superscripts are syntactic constructs, and naturally decompose into two identifiers in a specific relationship (even if that relationship cannot be further specified without going deep into some domain of discourse) -- and that is is much of the motivation for wanting to use them. "x2" does not carry that load.
Note that Unicode itself considers them *compatibility* characters and says:
Superscripts and subscripts have been included in the Unicode Standard only to provide compatibility with existing character sets. In general, the Unicode character encoding does not attempt to describe the positioning of a character above or below the baseline in typographical layout.
In other words, Unicode is reluctant to guarantee that x2, x², and x₂ are actually different identifiers! It's considered bad practice to treat them as the same, but not actually forbidden.
At least 2 technical reports (#20 and #25) discourage their use except in the case where they are letter-like (phonetic transcriptions use several such letters, where they have different meaning from their compatibility equivalents).
The more I look into this, the more I think it is really problematic.