The thing to realize about Apple is that the OS X installers are available but Apple just makes it very inconvenient to click the right buttons and get to it. They try to use their app store and so forth to funnel people to the latest release. But you can still find older OS X installers, directly from Apple if you look hard enough. As I said several times, you can use Dosdude’s Patcher, without actually patching anything, it has a menu item to download the relevant installer from Apple directly, bypassing the App Store. If you’re in catalina and you want to obtain the mojave installer, and presuming then App Store is funneling you away from mojave, then grab dosdude’s mojave Patcher. Find the download menu item and download the installer. Then quit the Patcher and run the installer.
Yes this is all rather confusing I agree you can complain to Apple about that but I don’t think they will care because they want it to be easy to be on catalina, and a PITA to lag behind. In any case you can obtain older versions of OS X anytime you want. It’s just not always straightforward from the App Store.
5,1 MacPro 3.46ghz x 12 128gb ram, OSX 10.14, Logic Pro 10.4.8, Mainstage3, Cubase10, StudioOne, Reaper, DP9, VEP, VSL, too many plugins to list