The original Android implementation of IAP created the Dart wrapper objects, which are the public API for clients to interact directly with the Android SDK (rather than the higher-level abstraction of the app-facing package, which isn't always sufficient), directly from platform channel JSON serialization. The platform communication has since been converted to Pigeon, but since the JSON methods were public methods on public classes, we couldn't remove them without a breaking change, so they were deprecated instead.

We should, either the next time we are doing a breaking change to the Android implementation anyway, or just as a breaking change to do the cleanup at some point (I would not expect the vast majority of clients to actually be broken by the change, since they would have to have decided to directly used the JSON for some reason, so it should be pretty low-cost).