The app switcher on Android used to be a core part of the operating system, but with the introduction of gesture navigation, it's now a component of your device's home screen launcher. This means it can be updated outside of OS upgrades, and that's exactly what is happening on OnePlus devices.
OnePlus Launcher 4.4 is now rolling out on the Play Store, and it brings a shiny new Recents interface with it. Before now, the app switcher interface consisted of a simple row of application previews, with a name and menu button above each item.
Left: Old UI; Right: New UI
The update moves the app icons to the bottom of the screen, where they are more visible. The button for clearing all applications also now has a 'Close Apps' label, and instead of pressing the menu button above an app preview to perform actions (like opening in multi-window or pinning), you now simply hold down on the app. No more reaching to the top of the screen.
Other new features include a new app search feature (activated by partially-opening the app drawer), and tapping on the app icons in the Recents UI jumps to that app. If you haven't already received OnePlus Launcher 4.4 through the Play Store, you can install the APK from APKMirror to get the new interface.