For me that didn't work. The red badge on System Preferences disappeared, but it came back the next day. Doing a `defaults delete com.apple.preferences.softwareupdate LatestMajorOSSeenByUserBundleIdentifier` got rid of it permanently.
> sudo softwareupdate --ignore "macOS Catalina"
It uses the label in the software update catalog to filter out specific updates.
You can get rid of it with:
> sudo softwareupdate --reset-ignored