If there is absolutely no way to force darkness we may have to develop a way to detect if people have shaders off so we can kick them out of the game. Which is not something I want to do but if there is no other way around it then there is no other way.
Make a client-sided addon to check for the value of
$Pref::ShaderQuality, and send it to the server. If it's at least 1, let them play. Otherwise, let them spectate along with those that do not have the client addon.
just some serverCmd/clientCmd functions is all that would take