Minecraft wiki entry for The_Wild_Update, associated text: "Also, to clarify, the Deep Dark won't take up all of below y 0. Other biomes like Lush Caves and Dripstone Caves will be able to generate there too, it's just that Deep Dark can only be found below y 0. Grimstone is its natural stone ;)" – @kingbdogz on Twitter, February 17, 2021