For example, if I put that @ClientModifiable annotation on a setting that controls the server, and a connected client modifies the setting, will things crash? Will they get disconnected? Or does fzzyconfig automatically prevent them from actually making the change?

@ClientModifiable simply means that the client is given control authority to make the change. It won't crash the server any more than a "normal" setting with proper operator privileges. All it does is skip authorization checks. Because of that I recommend not using it for server-sided settings unless you don't care about randos in a server being able to modify it.

Very helpful thank you, I was confused and assuming it served the same purpose served the same purpose as @nonsync lol.