Hi 
@mrploppy, 
@jasoncarle, 
@ATC
Indeed, during the update 2020-03 
SoftwareUpdate.prefPane was modified.
And I just carried out several tests, it no longer takes into account "softwareupdate - ignore" macOS Catalina "and re-writes each time in /Library/Preferences/com.apple.softwareUpdate.plist the value" LastRecommendedMajorOSBundleIdentifier "deleted (by the command running in terminal).
It also rewrites each time in ~/Library/Preferences/com.apple.preferences.softwareupdate.plist the value "LastRecommendedMajorOSBundleIdentifier" deleted (by the other command running in terminal).
Instead of deleting these values, I simply modified them but nothing to do, they are nevertheless replaced with each update from Preferences System and the red badge returns.
I also tested by replacing inside com.apple.preferences.softwareupdate.plist Product ID 001-09493 (Mojave) with 061-78581 (Catalina 10.15.5) but without any effects.
When running the terminal command: softwareupdate --list the red badge does not appear but the value "LastRecommendedMajorOSBundleIdentifier" in /Library/Preferences/com.apple.softwareUpdate.plist has however been modified.
I have no definitive solutions yet.
Unfortunately, Terminal commands must be run at each appearance of the red badge.
	
	
	
		Code:
	
	
		sudo softwareupdate --ignore "macOS Catalina"
	 
 
The next line must be run with "sudo" (before there was no need)
	
	
	
		Code:
	
	
		sudo defaults delete /Library/Preferences/com.apple.SoftwareUpdate.plist LastRecommendedMajorOSBundleIdentifier
	 
 
The following lines do not need "sudo"
	
	
	
		Code:
	
	
		defaults delete com.apple.preferences.softwareupdate LatestMajorOSSeenByUserBundleIdentifier
	 
 
	
	
	
		Code:
	
	
		defaults write com.apple.systempreferences AttentionPrefBundleIDs 0
	 
 
	
	
If you want to execute everything in a single command with copy/paste:
	
	
	
		Code:
	
	
		sudo defaults delete /Library/Preferences/com.apple.SoftwareUpdate.plist LastRecommendedMajorOSBundleIdentifier && defaults delete com.apple.preferences.softwareupdate LatestMajorOSSeenByUserBundleIdentifier && defaults write com.apple.systempreferences AttentionPrefBundleIDs 0 && killall Dock