With the lighting rewrite in 1.14 allowing for directional opacity, it seems that lighting as a whole just gives up if the player ascends any further than y=2048. This issue first appeared in 18w43a, a few snapshots before the directional lighting change. Above 2048, everything is dark, below -2048 appears to behave as full daylight instead.
For the horizontal axes this limit is 33,554,432, which is impossible to encounter without mods (but is incredibly annoying with mods).
Expected behaviour would be that light functions correctly above 2048 and beyond 33554432, out to the 32-bit limit (or perhaps even theoretically beyond) on all three axes, which it does in 1.13.2 and before.
If fixed, I would recommend moving the default world border out to 2 billion blocks, since with this out of the way, there should be no more game-breaking bugs out at this distance.
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Hello again!
This partially duplicates MC-168608. You may want to change your report slightly.

Shouldn't that one be forward resolved into this since it isn't explained in depth?
Okay, I put in a request 🙂
for the last sentence, 2.1474836 billion
Gonna be super honest here, it would be amazing if this bug was kept as a 'feature'. Sort of a "Hey, congratulations for making it this far. Your reward is an inhospitable land shrouded in darkness. Good luck (: "
People would start calling the far lands the "dark lands", players of anarchy servers could stretch for this goal for the ability to create a base/faction shrouded entirely in darkness, it could probably fit into some non-existant lore or something- Or, heck, maybe it could be named its own biome name!
It would be cool to have a dark lands, but if they were to keep that as a feature I still think they should push it back quite a lot

Can confirm in 1.17.1.

Even if this bug isn't fixed soon enough, Mojang should be able to move the border to up to 32,768,000 blocks as opposed to the 30,000,000 limit. If this bug does finally get fixed for once, my hope is that the world border should get to 262,144,000 blocks at least, or up to 1,074,741,824 blocks depending on whether or not they've patched whatever bug that causes massive amounts of internal server lag when farther out than around 200 million.
Affects 1.20.4. To be specific, the exact y-coordinate where this occurs is 2046.380 (presumably where the player's eye-level/camera is put 'inside' the area which causes this issue)
This can be observed using these commands:
/tp ~ 2046.380 ~
/tp ~ 2046.379 ~

Can confirm in 1.21 for the y-axis bug.
Hi there!
I can confirm.