In which apps is this the default? I can't think of a single one where a swipe were used to navigate "back" and "forth". Gestures are everywhere, but never in the capacity of the back button.
What do you mean? Flagship Android phones haven't shipped with a physical back button in years. Gesture navigation has been the norm for quite a while. And it works when apps use that system for its navigation patterns, which in my experience is every app on Android, including 3rd party.
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.
Do you use Android with gesture based navigation or with the old style 3 buttons? With gesture based navigation, swiping back is the same as the old back button in any app.