OK, I understand that the initial sentence is too strict, however, using codepoints for text editing and cursor position is even worse - even in your example of Korean there's a clear distinction depending on how the same character is encoded (combined NFC or not), but it should be the same to the user; and obviously if someone inputs a latin-diacritic character by pressing a modifier key before the base letter, then backspace removing the diacritic (since unicode modifiers are after the base letter) would be just ridiculous.

Backspace in general seems to be a very difficult problem because of subtly incompatible expectations depending on the context, as 'undo last input' when you're typing new text, and 'delete previous symbol' if you're editing existing text.