This is an easy Google search, but I'll add it just in case someone hasn't done it yet. If you'd like to avoid upgrading to Catalina until you think it's ready, but still get security updates, etc., run this in your terminal:

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

This ignores the system update, and gets rid of the red update dot.