The feature arrived along with several others in Android Auto 11.4
Now Android Auto will let you know which of your apps will only work when your car is parked.
Previously, if you tried to open an app that could only be used while parked you’d get an error message; however, you couldn’t tell from the home screen that was going to happen.
With the most recent version of Android Auto, now those apps are designated with a small P icon on the bottom right side of their app icon.
