The bug

In sufficiently large caves or other enclosed underground regions, distant unloaded chunks allow the sky to be seen as if the player were aboveground. This not only makes no sense, breaking immersion, but also allows for things like the time of day to be seen, which makes no sense for the underground without devices like clocks.

How to reproduce

1. Enter a sufficiently large underground region larger than the current render distance
2. Note that the sky can be seen in the distance

Expected result

Either the sky would be dark to match the darkness of the caves and make it not look like the sky in the first place (with the moon, sun, stars and clouds also not being visible at all through some dedicated visual mechanic), or it would be obscured by fog.

Actual result

The sky is clearly visible, despite the place in question being completely enclosed,

How to fix

Since biome json files can change the sky color of their associated biomes, the lush caves, dripstone caves and upcoming deep dark biomes could have their sky color set to something dark and different from the actual sky as seen from the surface.

As for other underground caves not associated with one of these biomes, a generic cave biome "minecraft:caves" could be added, which also has a dark sky color.

This solution would also fix other issues such as MC-214830 (since there'd be no interruption from aboveground biomes anymore) and MC-215387 (since fog color can be customized by biome jsons just the same). It could also allow for the fixing of MC-118103, as this hardcoded sky darkening would no longer be necessary if the biome can cause it rather than the Y level.

Currently there's no way for biome jsons to change the visibility or presence of clouds, the sun, the moon or stars, so such an option could be added to the biome format if needed.