Disabling custom sky and fog density from config file does not work. Is there any way to disable these CLIENT side features

Hm, I tested with customSky, and changing the value from true to false in betterend/client.json disables our custom renderer.

Fog-rendering is now controlled in bclib/client.json using the rendering.customFogRendering.

After some weirdness and the config cloning itself for some reason they started working

You probably know this, but just to be safe. You need to restart Minecraft when the config is changed.