In the current new snapshot 21w37a, I started a new world. When I entered the Nether, things appeared as normal. But as I explored, I found fossil block structures on Netherrack, not soul sand/soil. I hit the F3 and sure enough, I was in the Soul Sand Valley. As I was exploring further, I noticed the red crimson vines hanging high from the ceiling, so I figured the Crimson Forest was just ahead. When I got there, no trees, no crimson nylium, or any other red stuff. Just Piglins, Hoglins, and Netherrack. So it appears that the Nether Biomes are lacking their Biome specific surface blocks, and in case of the forest biomes, the trees as well.