When you go down near Y=16, you are still able to spot some deepslate patches. This was changed on Java 1.18 however, which breaks parity.
Step to Reproduce
1. Go find an open cave that goes all the way down to negative Y.
2. Go to Y=16 and check if there is any deepslate patches.
No deepslate above Y=0.
Deepslate patches still generating above Y=0.