The compatible line actually has nothing to do with custom skies. Just ignore it. If it truly bothers you, open the pack.mcmeta file in the texture pack and change the pack format number to "4". I've left it at an older pack number just to keep it universal.

If you're having issues like it's not working (can't see the sky) then there is something else client side. So far the thing that seems to fix the issue the most is to just reinstall optifine.

It's not your fault, they changed it so the "skip add" is harder to see. Here's the direct link to make it quicker for you: https://www.mediafire.com/file/m1h2ty2sqb4ydzz/Cygnus_Wall_Universal.zip/file

No. It requires optifine, but also optifine in 1.13.2 changed the way optifine works. See here how to fix texture packs for 1.13.2: https://www.youtube.com/watch?v=SvkWw_xKNYM


Eventually i'm going to make the pack universal so it works with every version of optifine.

It works just fine in 1.12 or 1.13. If it bothers you that it says incompatible, you can just open the pack.mcmeta and change the pack type from 2 to 4. That's the only difference between older skies and new skies is that pack type.