Is it this easy to remove the "pop up" prompt to install Catalina?!

... of course, that will mean that you will not get any macOS Mojave updates either.

There are, potentially, three other methods to disable this prompt:

• Option #1
  • Go to /Library/Bundles/
  • In the /Library/Bundles/ directory, look for "OSXNotification.bundle," and either move or delete it.
• Option #2
  • Download the macOS installer. When the download is finished and the installer window opens, quit the installer. You will find the Installer in your Applications folder. Simply leave it there as the app store background process checks your hard drive to check for whatever should be updated. It will notice the installer and you will not get another nag.
• Option #3
  • Enter the following command in Terminal: sudo softwareupdate --ignore "macOS Catalina"

Yes, you are correct on both.

Option #2 no longer works. It did in the past upgrades but this time unless Catalina is actually installed the red prompt remains telling you to update.

Does not work. next time you start up the prompt will return.