Apple used to make you go out of your way to download an OS upgrade. Then, Apple started having those OS upgrade installers auto-download to the /Applications folder. Then Apple made it so OS upgrades appeared as regular updates.
For Mac admins who aren’t ready to have their clients upgrade to Catalina (and potentially have a lot of things break), there is a way to tell the client machine to ignore the update and thus not advertise it to the user.
For those who are Munki admins, I’ve created a nopkg that will allow you to “install” ignoring the Catalina upgrade (but you can easily tweak the script to ignore any update) and also “uninstall” ignoring the Catalina upgrade (or any update).