When loading a world created in 1.16.5 (and later updated to 21w40a dirrectly) in snapshot 21w43a , chunks generated in 1.16.5 still have the bedrock layer and no deepslate beneath it and a deepslate wall between chunks generated in this snapshot.

Reproduce:
1) Open minecraft in snapshot 21w43a
2) Open a world created in 1.17.1 or bellow with chunks generated in those version
3) Check layers bellow y0 and the borders between chunks generated in this update