https://www.cnet.com/news/motorola-schools-android-p-on-how-to-make-swipe-navigation-good/

But if you want to go Back, you have to press a separate Back button to do so. Android P's Back button supports apps that require it, and it only appears when you can actually use it. Otherwise it doesn't show up.

This change is interesting and important because Android P will bring Android phonemakers a unified look and feel that they can riff on. Android P will also be the basis of 2018's Google Pixel phones, and of phones that run the more "pure" Android One operating system, which is about as Googly as you can get without being a Google phone.