This is a tool that snoozes the "App is Running in the Background" notifications on Android Oreo.
Judging by the relevent AOSP code (updateForegroundApps() in ActiveServices.java), there is no way for non-system app developers to suppress this notification when their apps are not on top.
However, NotificationListenerService can be used to snooze this notification for an extended period of time.
- Install from Google Play, or download and install the APK from here.
- Run the app and grant Notification Access permission in system settings.
- Start the service and manually snooze the "Running in the background" notification once. The app will keep it snoozed afterwards.