Apple - How to block updates to Mac OS X Catalina?

You can not block major updates after 10.15.5

• https://support.apple.com/en-us/HT210642

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)
• Run 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.)

if your OS version is "mojave 10.14.6", you will get the displays:

Ignoring software updates is deprecated.

The ability to ignore individual updates will be removed in a future release of macOS.

And I has fixed it by run

defaults delete com.apple.preferences.softwareupdate LatestMajorOSSeenByUserBundleIdentifier
softwareupdate --list

^{(Taken from https://tinyapps.org/blog/202005070700_remove_catalina_notification_badge.html)}

System Preferences -> Software updates -> Advanced. You will see options to toggle download and update choices.

It's hard to delay notifications more than a day.