You can not block major updates after 10.15.5
Before that update (and similar Summer 2020 updates to older OS - namely the 2020-03 update for High Sierra and Mojave, this command no longer works), you can ask Apple’s Software Update to ignore specific available upgrades to macOS Catalina:
- Open Terminal (in the Utilities folder in the Applications Folder)
sudo /usr/sbin/softwareupdate --ignore "macOS Catalina"
- Enter your login password if asked, hit Enter and you should get confirmation that “macOS Catalina” updates will be ignored.
Later, when you decide you want to allow the updates again, you can use:
sudo /usr/sbin/softwareupdate --reset-ignored
(With the Software Update preferences pane open, you can see the effect straightaway.)