There are suggestions for turning of the incessant upgrade notification here: https://discussions.apple.com/thread/250711218?page=1
Being on Apple Discussions I expect it will soon be removed by moderators so here is the gist of it:
From Terminal:
sudo softwareupdate --ignore "macOS Catalina"
when you are ready to install it (if
sudo softwareupdate --reset-ignored
This will not clear the red counter in the Sys Prefs icon in the Dock but will stop notifications.
It is also suggested that you go part-way through the Catalina upgrade process and halt it once the Install macOS Catalina.app file is in the Applications folder. Instead of that I just tried renaming the Mojave installer but that has not achieved anything. In any case both methods waste 8GB of storage.