Screen pinning is a fairly unknown feature despite the fact that it's been around since 2014. It's a helpful security tool that limits access to your device to only one app — perfect for those times when someone asks to borrow your phone. While previous versions kept this feature exactly the same, Android P's new multitasking UI changes the way it works.
Android P introduced gesture navigation. With this change, Google made adjustments to the UI, including a major revision to the recent apps screen. Because of this, Google knew that functions involving the recent apps screen would need to revised, including screen pinning. So for those of you who even know about the feature, here's how screen pinning works in Android P.
https://android.gadgethacks.com/how-to/use-screen-pinning-android-p-lock-apps-foreground-0184649/