Another day, another crack in the garden wall.
I'd like to see how Apple does this - Apple will have to track payments through alternative systems it has no control over, and then try to claim their 30% Apple tax from the developers for transactions it claims has happened. They'll have to include even more intrusive spying tech into ios to do this. I don't see how they'll do this without upsetting both the users and the developers.
They seem to be lower profile these days, but traditionally enforcement of contracts like this have been through the Business Software Alliance or similar groups. Search for "Business Software Alliance audit" and you can find plenty of information.