To stop MacOS from updating to Catalina--and stop reminding you about it:
sudo softwareupdate --ignore "macOS Catalina"
To clear this and begin receiving future updates:
sudo softwareupdate --reset-ignored
src: [https://twitter.com/mistydemeo/status/1191509386776068098]
THE REAL THING I WANTED TO SOLVE: Getting rid of that damn red notification circle on System Preferences
defaults write com.apple.systempreferences AttentionPrefBundleIDs 0
Restarts Dock
killall Dock