> of debug mode always doing checked div and checked add ops. this prevents a large class of issues which you can find in debug mode. In release mode those UB checks are dropped

Checked divs are present in release mode too. Omitting them would be UB.

Other checks (e.g. for additions) are omitted in release mode, but they were not preventing UB in the first place because they guard a wrapping addition, not an unchecked one (the kind that's UB in C).