ignore macOS update

 sudo softwareupdate --ignore 'macOS Big Sur'

Note: on catalina if mac is not in DEP, this will only ignore OS updates but not OS upgrade and this command is depreciated in Big Sur

sudo softwareupdate --ignore 'macOS Big Sur'
defaults delete com.apple.systempreferences AttentionPrefBundleIDs
defaults delete com.apple.systempreferences DidShowPrefBundleIDs
defaults write com.apple.preferences.softwareupdate LatestMajorOSSeenByUserBundleIdentifier com.apple.InstallAssistant.Catalina
defaults write com.apple.preferences.softwareupdate ProductKeysLastSeenByUser 061-21551
killall Dock

Ref:

https://forums.macrumors.com/threads/system-preferences-software-update-badge.2204123/page-2

Above method does not work 

The following works

add mac to MDM or set up a macOS server and enable profile manager, then install profile on targeted mac, Once profile installed, use following in terminal

softwareupdate --ignore "macOS Big Sur"

to verify if mac is enrolled in MDM or DEP USE FOLLOWING command

profiles status -type enrollment

Remove system update notification icon in macOS

Remove system update notification icon in macOS

defaults delete com.apple.systempreferences AttentionPrefBundleIDs; killall Dock