Minecraft 1.21.1
Neo-Forge 21.1.28
MATC 1.7.0

When attempting to use this on a serverit crashes. Claims its not for server usage.

latest.log

Yeahhh I'm going to have to say no to that one
seems like scam there

confirmed/ran into this as well today, calling clientside class on the server.

Is this ever going to be fixed?

yep getting this as well

Config screens cannot be registered on a physical server (dedicated).

In this case you can include the Dist argument in the Mod's class constructor and check it.

import net.neoforged.api.distmarker.Dist;

...

    public MATC(IEventBus bus, ModContainer container, Dist dist) {
        container.registerConfig(ModConfig.Type.CLIENT, MATCModConfig.CLIENT_CONFIG);
        container.registerConfig(ModConfig.Type.SERVER, MATCModConfig.SERVER_CONFIG);

        if (dist == Dist.CLIENT) {
            container.registerExtensionPoint(IConfigScreenFactory.class, ConfigurationScreen::new);
        }

        ModRegistry.register(bus);
        CREATIVE_MODE_TABS.register(bus);
    }

Apologies for the long wait, Github decided not to send any notifications for this issue. It should be resolved as of verison 1.7.1.