• Biomes have become immutable data classes
• Instead of using Biome objects, you now have to use RegistryKey<Biome> for most operations, since it will be stable even across world and datapack reloads
  • This means that any custom data you previously attached to your Biome subclass can no longer be attached directly to Biome objects, since Vanilla will serialize and deserialize Biome objects to copy them using their codecs, you could in theory attach custom data by extending the Codec, but this would mean this data would be attached to every Biome.
  • What you can do instead is maintain an IdentityHashMap<RegistryKey<Biome>, YourCustomData> in your mod, and query the custom data based on the biome's key.
  • The key of a biome must be obtained via the DynamicRegistryManager of the current world, ClientWorld and ServerWorld offer a convenience method for this called method_31081 (currently unnamed), which retrieves the key of the biome at a given BlockPos, although you can also do this yourself via world.getRegistryManager().get(Registry.BIOME_KEY).getKey(<the_biome_variable>).
• FabricBiomes#addSpawnBiome was removed, since this is now a property that can be set via Vanilla's Biome.Builder (see SpawnSettings.Builder#playerSpawnFriendly).
• Parent Biomes are gone and now hardcoded in AddHillsLayer#field_26727 based on their raw ids. Since the mapping is from unmodified biome → modified biome, there's no real point in exposing this to mods, because there can only be _one_ modified biome for each unmodified one.

The following diagram shows various objects that participate in defining world generation, and where they are registered/defined.

The general rule of thumb is that Registry will contain objects that contain some form of custom code, while BuiltInRegistry contains built-in data objects that mostly reference code objects from an associate Registry by ID and sometimes add configuration data.

As an example:

• Vanilla registers an instance of the class OreFeature under the ID minecraft:ore in Registry.Feature.
• Vanilla then registers 21 configured features in BuiltInRegistries.CONFIGURED_FEATURE that reference minecraft:ore, and configure it differently (for example minecraft:ore_iron to place iron ore)

For use in an actual world, all data objects from BuiltInRegistries will be copied by value into a new DynamicRegistryManager and may be overridden by JSON files in enabled data packs.