After checking the achievement notification, I was presented with this:
Context: I still have an account after using PDS to alter/remove all my posts and wiping my profile, but I still get messages from redditors seeking help so I check in from time to time.
Yep. Not even always ads exactly, but just dummy notifications to make your dumb brain keep opening the app or website (which is kind of a self ad).
Most apps I have to disable or heavily tone down notifications in some way to get them to stop yapping about nothing. Even then, sometimes you can’t get notifications you do want without junk.