Subscription models only make sense for an app/service that have recurring costs. In the case of Lemmy apps, the instances are the ones with recurring hosting costs, not the apps.
If an app doesn’t have recurring hosting costs, it only makes sense to have one up front payment and then maybe in app purchases to pay for new features going forward
You’re the one speculating. You can analyze your network traffic to ensure it disabled, and as people have done and verified that it is disabled. Those are standard Google Ad trackers. Any app with ads has them, like Sync for Reddit did and Sync for Lemmy does.