User killing is different from the OS killing. And push will not (at least should not) restart the app in this case. You can see it with adb logcat where GCM (or ntfy) intent will be cancelled by the OS.
Also by default ntfy.sh got low limit, so I’d recommend to use your own up server if possible (else notifications may not arrive). You can check with the url given by the ntfy app. If no new notification arrives on this topic, it’s probably rate limited.