Many app need to keep state while they are in background though. Imagine your browswer tabs reloading every time you swotch to a different app and back.
Also Android has ongoing notifications which let your app stay running forever and iOS introduced Live Activities in iOS 16 which can be used similarly (e.g. Instagram uses it to finish uploads in the background). But what the author of “Don’t kill my app” wants is to run in the background without making it obvious to the user, which many would consider malicious.
https://developer.apple.com/documentation/uikit/view_control...
https://developer.android.com/topic/libraries/architecture/s...