It seems like an effort was made in 21w38a to remove all old worldgen files.
Since some biomes are no longer used in vanilla, these biomes could be removed as well.

In case upgrading old buffet worlds is a concern, the hill biomes at least are functionally identical with their non-hill variants, so the biome ids could be datafixed in that case (desert lakes and regular desert only have the difference that desert lakes generate fossils, mushroom shore and mushroom islands are also identical)

The following biomes are currently unused by worldgen:

• badlands_plateau
• bamboo_jungle_hills
• birch_forest_hills
• dark_forest_hills
• desert_hills
• desert_lakes (see MC-236852)
• giant_spruce_taiga_hills
• giant_tree_taiga_hills
• jungle_hills
• modified_badlands_plateau
• modified_gravelly_mountains
• modified_jungle
• modified_jungle_edge
• modified_wooded_badlands_plateau
• mountain_edge
• mushroom_field_shore
• shattered_savanna_plateau
• snowy_mountains
• snowy_taiga_hills
• snowy_taiga_mountains
• snowy_tundra
• swamp_hills
• taiga_hills
• taiga_mountains
• tall_birch_hills
• wooded_hills

Steps to Reproduce

1. Download https://github.com/slicedlime/examples/raw/master/vanilla_worldgen.zip
2. go to worldgen/biomes and look for definitions of the above biomes

Observed Behavior

They are still there despite being unused and identical to other biome definitions

Expected Behavior

Prune the definitions that are not needed anymore.