In experimental worlds with Caves & Cliffs turned on, mobs do not spawn at y=0 or below. This affects basically everything but the wondering trader as far as I have seen.

How to reproduce:
1. Create a new world on hard mode with C&C turned on
2. Clear out a large area at Y=0 or below, and observe
-Move around a bit to despawn mobs that will spawn above you and take up the mob cap

You won't see anything spawn at Y=0 or below

Expected Results
Mobs should be able to spawn in these portions of the world

Tested in a world created in the beta, then upgraded to caves & cliffs via experimental mode.