I managed to disable the fog by manually setting the unity_FogParams.xyz to 0. (see documentation)
unity_FogParams = half4(0.0, 0.0, 0.0, 1.0);
Maybe it’s because FOG_LINEAR (as well as the other 2) is a global keyword so we cannot undefine it like a variable?
The custom function can be linked to any fragment block.
