How to disable major release upgrade in Mojave?
Supplements update are always better to have it and it will come automatically under Software Update if you press Check for update. You can choose whether to install it or not. 

However, you will never get Catalina from Software Update if you are running Mojave. Catalina will be available in macOS store later as Apple has done for past OS. Later you download it if you want to install else for the whole year, Apple will never force to update you from running OS to a new OS. It's totally up to you.