Happy to see an open-source shader! This allows me to now have every part of minecraft besides the actual game open sourced, and I consider this an accomplishment.

With that aside, I'd like to say that using this shader (on any preset) does now allow for a proper day/night cycle in the overworld, and instead keeps the skybox in the "day" position. I have not tested results in the nether or end but I can upon request. I'd also like to add that shadows don't seem to render at all.

(I can give screenshots upon request.)

System specs and other useful information:
Minecraft 1.16.5
MakeUpUltraFast latest version
Iris latest version
Sodium latest version
Indium latest version
AMD Ryzen 5 3600, 32GB of ram
NVIDIA GTX 1650 SUPER with proprietary driver
Arch Linux

Edit: I'd like to add that this is different from #21

I'm not clear what you're talking about.

MakeUp has two branches in Github. One doesn't work with Iris. The "master" branch only works with Optifine. The Legacy branch works with both. Please check and specify which version you are referring to.

If in doubt, use the CurseForge version. That's the one that works with Iris. I think you must be using the wrong MakeUp branch.

It's more useful if you say something like:

Expected result: Blah, blah, blah.

Result obtained: Blah, blah, blah

This does not happen in the nether:

I'd also like to add that godrays cause a visual bug when they are on at all. See below:

This is proof of the day/night skybox bug from MakeUpUltraFast version 8 legacy:

Intended behavior: Dark/Night sky box.
Actual behavior:

For comparison, this is day:

OK...something even weirder happened:

after some time, it did become night, but it seems to not be based on the in-game day/night cycle. Therefore, as in the screenshot, i can set it to day and still have the night skybox, and vice versa. Maybe the day/night cycle skybox is hardcoded to fade at a specific time?

No problem. I understand that it is a bit confusing. The Legacy version WAS for older versions of Minecraft. Now there is a unified release for all versions in Optifine and Iris.

The problem is that to work in Iris you have to make special settings that will be removed when Iris incorporates all the functionality of Optifine. So, the "legacy" branch has those special tweaks (which will be abandoned some day), and the "master" branch will be the final version (but, for now, it only works in Optifine).

You are using the version for Optifine. Use the version for Iris (legacy branch). Or download the CurseForge version.

edit:
I apologise.

I was using the last legacy release (as hinted in the documentation that this is for iris) and it had this bug. I did not bother trying a newer version because I play on 1.16.5 and i didn't know whether the 1.19 shader would work in 1.16.5. As of version 8.6, everything works fine. You are right, but there was a miscommunication.