https://www.cnet.com/news/what-google-android-p-can-learn-from-motorola/

However, the Back button remains. If you want to go back within many apps, you'll need to press that legacy 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 even show up).

This change is interesting and important because Android P is Google's next Android operating system for fall. It brings 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.