Is onPause guaranteed to be called when an Activity is no longer running?

If my app crashes

onPause() is not called here, but this shouldn't bother you as your entire app process ceases to exist, inclusive of all threads you have created (UI or otherwise).

Activity somehow ceases to exist

onPause() will be called whenever your Activity is moved to the background from the foreground, which is done in every conceivable way in which your Activity can be shut down, except for your app crashing. However, as I mentioned above, the app crashing will also by default kill your thread.

onPause() is essentially called whenever your Activity is no longer in the foreground. Your Activity may still be alive and in memory after onPause() has been called, but there is no scenario which I can think of in which your Activity is alive and in the background without onPause() being called.

Yes , onPause() will be called when an activity is no longer running. Suppose an activity is closed then the sequence of events will be onPause() -> onStop() -> onDestroy(). Even if your activity slips into the background when another activity starts or when the screen switches off, onPause() is always called even if the other two methods aren't called. So even if activity ceases, onPause() will be called and your thread will be killed.

But when your app crashes, along with your entire activity, even the thread that you have started will be taken care of by Android by finishing it all.