I don’t see how UTF-8000 doesn’t have it. The first byte of any code point is either 0xxxxxxx or 10xxxxxxx, which is distinct from all non-first bytes which are

I don’t see how UTF-8000 doesn’t have it. The first byte of any code point is either 0xxxxxxx or 10xxxxxxx, which is distinct from all non-first bytes which are 11xxxxxx. Thus any UTF-8000 sub-bytestring must necessarily have the start aligned at a code point boundary, at which point all the subsequent bytes are interpreted as codepoints in the same way.