The bug
When the camera is below minimum world height (feet at about y = -65 by default), the bottom half of the sky suddenly goes dark.
How to reproduce
Generate a void superflat world, and fly down to the void. The lower portion of the skybox will abruptly flash to black.
Code analysis
Based on 1.11.2 decompiled using MCP 9.35 rc1
This happens when the player eye level is below net.minecraft.world.World.getHorizon()
which might be done to not have a blue blackground when you are in caves. It would be good if this effect was achieved differently.
Linked issues
is duplicated by 16
relates to 1
Attachments
Comments 30

Can confirm for MC 1.12.1.
Apparently happens at y=0 now, 20w07a.
Can still reproduce in 20w10a, but not on superflat worlds.
This also happens in buffet worlds and in the ocean.
Not really, when below the world, the color abruptly changes still; when in caves, it's the normal sky color (not darkened), etc.
There's more than 1 aspect to this bug.

Ah, I see. So the boundary is effectively at y=-64 now instead of y=+63, but the bug still exists? I'll update the report accordingly.

I will say that in earlier versions flying below minimum Y had its own darkening effect, which is the residual stuff from void fog. It's distinct from the black skybox of the void.
can confirm in 1.19.4 and 1.20-pre7

Confirmed in 1.21.4 prerelease 1.
@@unknown I changed the report a little bit, I hope you are fine with that.