Would you like signs with those chars?

> they do that fine on UTF-8 data without any decoding.

No they won't, at best they will pass through non-ASCII without doing whatever the function is defined to do (e.g. tolower won't actually lowercase your letters), and at worst they will silently corrupt it (if they think it's one of the legacy 8-bit encodings).

> You often do not need the fancy Unicode is_digit but only is_ascii_digit for real software because overwhelmingly the "is it a digit?" question is not cultural but purely technical.

There are a subset of edge cases where a string does not contain linguistically useful information, like a phone number or UUID. In those cases, these ASCII-only functions are somewhat useful, but most of them could just as easily be done with regular expressions like [0-9]+. Realistically, you need nontrivial parsing logic anyway, to deal with things like embedded dashes and other formatting vagaries, so you may as well solve both problems with the same tool (which can and should be Unicode-capable, because ASCII is ultimately "just" a subset of UTF-8). In that context, these ASCII-only functions look rather less useful to me.

The problem is, ASCII-only functions are also an attractive nuisance. They make things a little too comfortable for the programmer who's still living in 1974, the programmer who still thinks that strings are either ASCII or "uh, I dunno, those funny letters that use the high bit, I guess?" Those programmers are the reason that so many Americans "can't" have diacritical marks in their names (on their IDs, their airline tickets, etc.). If you are writing string logic in 2022, and your strings have anything to do with real text that will be read or written by actual humans, then your strings are or should be Unicode. Unicode is the default, not the "fancy" rare exception. If you have strings, and they're not some variety of Unicode, then one of the following is true:

1. They're encoding something that sort of looks like text, but is not really text, like a phone number.
2. They are raw bytes in some binary format, and not text at all.
3. In practice, they mostly are Unicode, but that's not your problem (e.g. because you're a filesystem and the strings are paths).
4. You hate your non-English-speaking users (and the English-speakers who have diacritical marks anywhere in their string for whatever reason - we shouldn't make assumptions).
5. You inherited a pile of tech debt and it's too late to fix it now.