Skylight drops to 0 down in the Grim Depths (negative y levels)

If you dig a hole down to bedrock, light from the sky can reach all down and still illuminate the depths as bright as day. Now that the Overworld has gotten deeper, it should get darker as well.

The Grim Depths

The Overworld is mostly stratified into 3 main layers: the thin Dirt Layer on top, middle Stone Layer, and the Bedrock Layer at the very bottom. 1.17 extended the world depth even deeper and introduced a new layer where stone gives way to grimstone. For this reason I'm calling this zone between bedrock and stone the Grim Depths. This is to prevent confusion with the Deep Dark, which generates in this layer.

Where stone transitions to grimstone, the skylight level starts dropping from 15 to 0. The sky will also fade from blue to black. If you opened up a tunnel to the sky, or the roof is so far away it doesnt render, you wouldn't see the blue sky from the Grim Depths, its pitch black down here. The only light you get is the light you bring like torches.

The Deep Dark

This subterranean biome should still be especially dark. If you could look up and see the sky from down here, its so dark you'd see the stars in the middle of the day.

The Abyss

This change affect oceans as well. A new ocean biome called the Bottomless Sea generates with the seafloor in the Grim Depths. The Abyss is the layer of water in the Grim Depths where the skylight can't reach. If you swim down here and look up, the sky will appear black so you are truly shrouded in darkness.