Minecraft wiki entry for Development_versions, associated text: There is now experimental support for a worldgen folder in data packs.
worldgen/biome can contain biome definitions.
worldgen/configured_carver can contain definitions for world carver settings.
worldgen/configured_feature can contain definitions for feature placements.
worldgen/configured_structure_feature can contain definitions for structure placements.
worldgen/configured_surface_builder can contain definitions for surfaces.
worldgen/processor_list can contain sets of block processors.
worldgen/template_pool can contain pool definitions for jigsaw structures.
Custom biomes can now be used in the single biome/caves/floating islands world types after adding the data pack containing the biome first.
Custom biomes can now be used in custom dimension generators.