As I've tried updating my old world to the more recent snapshots (21w43 thru Pre-release 1.18), I am still seeing bedrock at the bottom of my world in the old chunks, in the old cave generation. There is no deepslate in place of the bedrock, nor is there any generation below y=0. Exploring new chunks does generate the new generation below y=0, but I can't see anywhere underneath my pre-existing chunks of anything new, even when viewing in spectator mode.