Beware installing macOS Mojave! Apple appears to have completely removed the ability to download the installers of older versions of macOS and Mac OS X from the new version of the Mac operating system.
Previously users could go to the Mac App Store, click on the Purchased tab and see a list of all the apps they had ever downloaded.
This was particularly useful if a Mac user wanted to revert to an older version of Mac OS X, or run more than one version of the operating system (perhaps for testing, or because they were running old software).
It was previously possible to locate an older version of Mac OS X in the Purchased tab, re-download its installer, and then follow various steps (outlined here) to create a bootable installer on an external drive. This bootable installer could then be used to do a clean install of the earlier version of the MacOS on the Mac.
Now that Apple has updated the Mac App Store in macOS Mojave, there is no longer a Purchased tab. Users can see some of their previously purchased items if they click on Store in the Apple Store and choose: View My Account. However, this will no longer show any older versions of MacOS, though it did previously.
In 2017, when Apple introduced MacOS High Sierra, the company did remove Sierra from the Purchased list, which made it more difficult to revert to that earlier version, but it was still possible to download the Sierra installer via this link that Apple provided
The link still takes you to a page for macOS Sierra, but if you are running Mojave you will see an “Update not found” error message if you try to download it.
We have checked the same link to the Mac App Store on a computer that hadn’t got Mojave installed and can confirm that it is still possible to download the installer.
We also tested the link Apple provided to download El Capitan back when High Sierra launched (here). But that download also no longer appears to be valid if you access it in Mojave, with the message “Update not found: The requested version of macOS is not available” appearing.
Again, we were able to confirm that El Capitan installer could be downloaded on a Mac running High Sierra and on one running Sierra.
To reiterate, it is possible to download the installer for an older versions the Mac App Store but only if you are running macOS High Sierra or older. If you are running macOS Mojave this will not be possible.
In fact, Apple even states as much on its page about creating a bootable installer, here. It says: "To download macOS Mojave or High Sierra for this purpose, download from a Mac that is using macOS Sierra 10.12.5 or later, or El Capitan 10.11.6."
This means that we have to recommend that you download the installer for High Sierra, and any other installer you feel you may need in the future before updating to Mojave.
To check to see if you have the installer you require on your Mac, perform a Spotlight search, for Sierra or High Sierra for example.
Luckily, we already have the various installers on our Mac. If you don’t have them available on your Mac then we recommend finding a Mac that hasn’t got Mojave installed, logging on with your iCloud ID and viewing the Purchased tab in the Mac App Store where you should be able to download the installer you require.