I’m just another forum member here, but I wanted to comment. I personally appreciate you taking the time to detail this, because I have some older projects that will face some of these issues.

I have posted about a similar issue here: https://developer.apple.com/forums//thread/693633?answerId=693655022 It is quite astounding that Apple doesn't have straightforward download links to old OS to test against, and that it seems that formerly posted links are now non functional (perhaps depending on the os version a system is running, unsure). This new Developer Site setup has nice changes, but it is not even close to as functional as it used to be, and I can't figure out any good reason for it. Kind of a "no one home" feel these days when it comes to issues like this, maintenance of documents, sample code, etc.

Any chance of Apple getting some working links to the operating systems up so that people can install them on systems for testing? Or is there some other way to get them?

Yes, these are the non-working links I refer to. When these links are clicked, the behavior I described occurs.

The links for past OS versions on the main Apple site trigger AppStore to open. Clicking “Get” in AppStore in turn causes System Preferences software update to open. Progress spin happens, then a message that the “file is unavailable” appears (paraphrasing…it may say “download unavailable”, something like that). I tried doing this in Safari as well as Chrome, and also seeing if the links directly led to downloadable files, but neither seemed to resolve the problem.