I fully agree. It's insane that Apple makes screens too big to use with one hand AND puts important controls like the back button at the top of the screen. Like you were saying, it's really frustrating that the reachability gesture triggers things in apps--e.g., Twitter's navigation menu completely overlaps with the reachability gesture area, so you can virtually never hit the "back" button because the reachability gesture will reliably send you to Twitter's "search" page.
It uses the accelerometer that has already been built in to iPhones going way way back (actually ALL iPhones have at least a 3-axis accelerometer), so that's why there is so much support for older devices.
It was added in iOS 14. Not hardware specific. It works on my iPhone 11 Pro Max. It's an accessibility feature that has to be specifically turned on though.
