Exactly what it sounds like.
This is exactly what it sounds like, "deprecated" means "going away". They can't just pull it, because then applications, that used it, wouldn't work. So essentially developers get told - we're going to pull this, make sure your applications aren't using it when we do.
So a developer can track a specific instance of their application, but nothing else. So you can track your applications on someone's phone (usage data for example) but you can't tell if (as an example) you have multiple applications installed by the user.


