Minecraft wiki entry for Development_versions, associated text: The dimension folder is no longer present in the vanilla world generation reference pack, but dimension files still work like they used to inside a custom data pack.
There's a new file type that the data report generator can output in the biome_parameters folder, containing all the biome parameters for the default vanilla dimensions.
Custom dimensions no longer require a separate seed field, and in fact can no longer have a custom seed - the world seed is now always used for all dimensions.