Note to self: Ignoring the Catalina Nagscreens is done like this:

$ sudo softwareupdate --ignore "macOS Catalina"
$ defaults write com.apple.systempreferences AttentionPrefBundleIDs 0 && killall Dock

And that should make MacOS ignore Catalina until further notice. Further notice does look like this:

$ /usr/sbin/softwareupdate --reset-ignored