It's not guaranteed for 7-bit ASCII either because tolower/toupper are locale-dependent and with the tr_TR lowercase I (U+0049) is ı (U+0131, aka dotless i) wich encodes as two bytes in UTF-8.
That's not ascii then. It's byte width compatible (to a certain degree as you point out). But it's not ascii. ascii defines 128 code points and the handling of an escape character. It doesn't handle locales.
