The custom sky textures don't work on Pre-3 Vanilla servers (servers, not singleplayer!).

The class 'ClientLevel' gets the effect through:

this.effects = DimensionSpecialEffects.forType(dwm.registryAccess().dimensionTypes().getResourceKey(cha));

which used the dimension data sent in Login and Respawn packets to get the dimension key resource location, but this doesn't properly resolve since 'DimensionType' does not implement hashCode or equals (this works on singleplayer due to them just being the same instance).

Possible solution: Either send the resource location in Login and Respawn or implement hashCode/equals in DimensionType (the first of which would be preferred for certain modmakers tho)