[Experimental] Deepslate patches still generating above Y=0

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.

Expected Result
No deepslate above Y=0.

Observed Result
Deepslate patches still generating above Y=0.