Not many people know that they are till available, but require a few shortcuts. AppleInsider tells us more. If you use the App Store on your Mac, know that once it downloads, it will complain that this version may not be able to be installed. Goto your Finder, and do a search for the installer. It should show in your Apps folder, but if not, you can find it and copy to a location of your choice. If you have an archive of all your old installs, they can expire, so feel free to grab them all again. Read for more information!
https://appleinsider.com/articles/19/11/23/how-to-get-an-old-macos-installer-from-apple
