Problem is, iPhone and iOS never had a back button. That was always left to developers to put in their apps. Apple did have guidelines for app creation, which is why the back arrow is almost always in the top left of the screen, but that is nowhere near as intuitive as a system-wide back button (or gesture).

This is one of those “it’s always been done this way” things that Apple probably won’t ever change, unfortunately. It was also my biggest gripe when I switched from Android a few years ago, and may be a reason I go back to Android.

Not illegal, but against their terms of service. Worst they can do is ban you; you’ll be fine.