I believe that as long as you have a privacy policy where you disclose how are you using their personal data(if any), you could use cloud-functions
to trigger on different events.
You could use FirebaseInstanceId.getInstance().getInstanceId()
in conjunction with FirebaseAnalytics.getInstance(context).getAppInstanceId()
to send messages to tokens or to subscribing them with a cloud-function
to a topic "news", like this you can even target anonymous
users of which you don't really have personal data.
Be aware that in order to use events to record user usage, you'll have to mark those events as conversion
events.
I personally use app_remove
and app_clear_data
to have my anonymous users removed from my project upon triggering any of those events.