How to block updates to Mac OS X Catalina? Mac OS X Catalina will be released in a few days, so how can I block updates to that OS so it doesn't get downloaded and, ideally, I don't even get prompts to upgrade.

I have 32-bit software I intend to keep on using, so I can't update since this version of OS X will support 64-bit apps only.
You can not block major updates after 10.15.5


https://support.apple.com/en-us/HT210642


Before that update (and similar Summer 2020 updates to older OS - namely the 2020-03 update for High Sierra and Mojave, this command no longer works), you can ask Apple’s Software Update to ignore specific available upgrades to macOS Catalina:


Open Terminal (in the Utilities folder in the Applications Folder)
Run sudo /usr/sbin/softwareupdate --ignore "macOS Catalina"
Enter your login password if asked, hit Enter and you should get confirmation that “macOS Catalina” updates will be ignored.


Later, when you decide you want to allow the updates again, you can use:

sudo /usr/sbin/softwareupdate --reset-ignored


(With the Software Update preferences pane open, you can see the effect straightaway.)