Relates to MC-204649

If a mob has its own footstep sounds (like zombies, spiders, skeletons, etc.), it won't make crunchy sounds when walking on deepslate blocks.
Probably WAI as most mobs are unaffected by the block they are on (MC-4826) and the related is a special case, but seeing as how deepslate replaces stone under y=0 it might make sense to fix.