Right now, potion types and potion recipes are still not data-driven, and instead must be registered through private methods that add to private fields. Fabric should probably add a way to register new potion recipes so people don't have to make lots of identical Invoker mixins.

This was implemented using transitive access wideners.

This would make a good PR. But it would probably be better to hook at the end of registerDefaults, and provide the necessary methods as lambdas to a registry impl class + registry API class exposing the registration methods.