Thanks to https://apple.stackexchange.com/questions/367995/how-to-block-updates-to-mac-os-x-catalina
Instructions for macOS 10.14 Mojave
Disable (ignore) Catalina upgrade notifications:
run sudo /usr/sbin/softwareupdate --ignore "macOS Catalina"
Remove Catalina badge icon from system preferences:
run defaults delete com.apple.preferences.softwareupdate LatestMajorOSSeenByUserBundleIdentifier
and then open System Preferences > Software Update
Enable (reset) Catalina upgrade notifications:
run sudo /usr/sbin/softwareupdate --reset-ignored
Sadly, no longer working 😞