How to know when my app has been killed?

there's no way to determine when a process is killed. From How to detect if android app is force stopped or uninstalled?

When a user or the system force stops your application, the entire process is simply killed. There is no callback made to inform you that this has happened.

When the user uninstalls the app, at first the process is killed, then your apk file and data directory are deleted, along with the records in Package Manager that tell other apps which intent filters you've registered for.

I have found one way to do this.....

1. Make one service like this

   public class OnClearFromRecentService extends Service {
   
       @Override
       public IBinder onBind(Intent intent) {
           return null;
       }
   
       @Override
       public int onStartCommand(Intent intent, int flags, int startId) {
           Log.d("ClearFromRecentService", "Service Started");
           return START_NOT_STICKY;
       }
   
       @Override
       public void onDestroy() {
           super.onDestroy();
           Log.d("ClearFromRecentService", "Service Destroyed");
       }
   
       @Override
       public void onTaskRemoved(Intent rootIntent) {
           Log.e("ClearFromRecentService", "END");
           //Code here
           stopSelf();
       }
   }
   

2. Register this service in Manifest.xml like this

   <service android:name="com.example.OnClearFromRecentService" android:stopWithTask="false" />
   

3. Then start this service on your splash activity

   startService(new Intent(getBaseContext(), OnClearFromRecentService.class));
   

And now whenever you will clear your app from android recent Then this method onTaskRemoved() will execute.

NOTE: In Android O+ this solution only works when the app is full-time in foreground. After more than 1 minute with the app in background, the OnClearFromRecentService (and all other Services running) will be automatically force-killed by the system so the onTaskRemoved() will not be executed.