Apple - 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.

To ignore all updates that start with "macOS Catalina":

sudo softwareupdate --ignore "macOS Catalina"

To reset all ignored updates:

sudo softwareupdate --reset-ignored

Source

For more details, check out man softwareupdate:

     --ignore identifier ...
                 Manages the per-machine list of ignored updates. The
                 identifier is the first part of the item name (before the
                 dash and version number) that is shown by --list.  See
                 EXAMPLES.

Unfortunately, this does not ignore only major releases to the best of my knowledge, but it will stop the nags until disabled. (I too am interested in the ability to ignore only 10.xx.0 major releases...)