Last weekend, I outlined the preparations I’m making to upgrade to Catalina, and recommended that if you intended running an older version of macOS in a Virtual Machine or dual-booting, now was the time to ensure that you downloaded the installer for that old version.
Al Varnell kindly pointed out that, in addition to Mojave which remains available to all Macs capable of running Mojave (and sometimes even those which aren’t!) at present, access to previous installers is still supported in the App Store, giving the following links:
Those links do take you to pages in the App Store for those older versions of macOS, but whether they will deliver them to your Mac isn’t as simple, I’m afraid. The basic rule appears to be that if you try to Get a version which is too old for that Mac to boot, then this is what you see when Software Update tries to get it for you:
So this iMac Pro, which shipped with High Sierra installed, can still download the Mojave installer, and that for High Sierra, but Sierra and El Capitan aren’t actually available despite being offered in the App Store. To get either of those, you’ll need to use a Mac which can boot in that version of macOS.
If you want to run some older versions of macOS in a VM, such as Yosemite or El Capitan, you’re also likely to be out luck, even if you can find somewhere from which to download it: in his worrying summary about this subject, Michael Tsai found that they won’t install because their certificates have expired. One workaround for that is to change the date on your Mac to one during that cycle of macOS, just for the purpose of installing.
If you’ve not found a solution yet, one place to check is 7labs, which offers detailed instructions on how to obtain High Sierra from official Apple servers, for example.
There are also various sites and one app which claim to deliver Apple installers hosted independently of Apple, and in some cases complete working VMs. I strongly recommend that you avoid using any of those: some could possibly be malicious, they’re good targets for attackers to subvert with malware, and they’re also a breach of Apple’s copyrights and licensing.
Whatever you decide to do, I still recommend that you do it now, before Catalina’s release. Experience tells us that certainly won’t make it any easier to obtain old macOS installers, and I suspect that even finding the Mojave installer in the App Store will become harder in a few weeks time.