Old MacOS versions can not be downloaded at this time, as Heise has tried, referring to reader feedback. The links are still there, according to Heise in the Mac App Store. However, there are error messages in the sense of unavailability during the download.
Therefore, it is also unclear whether it is simply a bug in the App Store or it is a strategy change on the part of Apple. With mobile operating systems, for example, downloading old versions is only possible within a short period of time. Anyone who makes an iOS upgrade and discovers an incompatibility with an app a few weeks later has had bad luck.
Oddly enough, some readers of Heise reported that the download of MacOS 10.13 from MacOS 10.14 succeeds. An official statement does not exist so far. Restricting downloads is difficult as it also affects users who need boot media from older versions. The days when Apple still settled the operating system as a CD on the computers are long gone. But that also applies to the Windows world.
Apple plans, as every year, to publish a new MacOS in the fourth quarter, whereby an operating system is taken out anyway anyway from the software support by Apple. This time, the users are well prepared. There is a list of software that will not work with MacOS 10.15 aka Catalina. It can be assumed that the developers will update their programs after a certain degree of stability of the operating system. With very old software and in particular 32-bit software is but in any case.