Attempting to create a world with the Caves and Cliffs part 2 data pack and then uploading the world to Minecraft Realms results in the world generation being reverted. The terrain generated in the single player world below y level 0 is removed when the world is uploaded to a Realm, so the bedrock level returns to y level 0, but the surface terrain relatively unchanged (same seed). This makes playing with the new cave generation not possible on Realms.