Deepslate does not generate accurate. It only generates in bubbles above y=0 and not under it.
When I went under y=0 I didn't saw any pice of deepslate and only above y=0 - 16 
Stone everywhere and 0% of deepslate fix this before official update
When I went under y=0 I didn't saw any pice of deepslate and only above y=0 - 16 
Stone everywhere and 0% of deepslate fix this before official update