If you enable gesture navigation, swiping from the side of the screen emits a virtual back button press. It's an OS level gesture (that apps can attempt to disable, but that can be overridden by the user). This doesn't require apps to implement anything it they implement normal back button behaviour already.

Several brands have removed button navigation by default. I think you can still enable it in the settings, but the button bar at the bottom of the screen seems to be the exception these days.