I am thinking of not wanting to upgrade from Mojave to Catalina because, so far my setup & configurations of different apps like web-dev stuff and mobile-app development are all working and in proper order. It has taken me a long time to get to this point. Fear of breaking apps - esp 32-bit ones (if any) in the new upcoming version of MacOS has made me think of not going for the upgrade. How / Where do I disable a major release upgrade ?
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.