If you're not ready/able to upgrade to macOS Catalina, you can hide the update from Software Update.

Note that because of sudo, you'll need to be an admin on your Mac.

Open Terminal and enter the following command:

sudo softwareupdate --ignore "macOS Catalina"

Enter your password when prompted.

When you are ready to update, you can un-hide Catalina the same way:

sudo softwareupdate --reset-ignored