I think it is related to the App Store (not a problem with direct downloads from Apple's servers), perhaps the old download pages have been pulled, and Apple will soon be using other links for those. Old systems, such as El Capitan and Yosemite, continue to be available, and going back further to Lion or Mountain Lion, use Support page downloads for those.
Alternatives, such as the MDS app, will also pull system installers from the Apple servers (back to High Sierra, as far as I can tell. I downloaded both Catalina and Big Sur via MDS today, just as a test.
By my experience today, it looks like the links for more recent macOS systems that are supposed to open the App Store -- don't open the relevant page on the App Store. Again, there's other methods for getting the system installers, if you really need one.