You need to have an old version of Pages, Keynote and Numbers installed in your Applications folder to be able to update them. If you lack these (either because you deleted them or never had them installed first hand) and never registered the purchase with your Apple ID, you need to work your way up through the missing years of upgrading, including the registration with your Apple ID to be offered an installation on any system that's older than the currently shipped Mojave:
- Install an old
iWork suite (from your original installation medium, or use the iWork ’09 trial suite that can still be downloaded from Apple.
- You now have an
iWork 09 folder inside your
Applications folder, containing trial versions of Pages, Numbers and Keynote.
- Launch Pages, Numbers and Keynote, then quit each app. (This step was suggested by the instructions based on the 2014 article. It doesn't seem to be necessary any more - I had only launced one of the programs.)
- The trial suite can be upgraded to
iWork 9.3 with a downloader available at Apple - Support - Downloads. I cannot verify if this step is necessary since I had installed that upgrade. I suspect it can be skipped.
- You still have an
iWork 09 folder inside your
Applications folder, containing trial versions of Pages (v4.3), Numbers (v2.3) and Keynote (v5.3).
- Launch the Mac
App Store, go to the
updates tab, and activate
show incompatible updates. Selecting one of those apps will take you to the app's page. There, select update. The store asks for purchase confirmation and your password (if required by your settings). Then it will attempt to update the application, and fail, telling you that they require 10.14. Repeat for all three (five, if you installed iMovie and Garageband from your original DVDs) applications anyway, since from then on this application will appear in your
Purchased tab. (Note: The
Purchased tab disappeared with
MacOS 10.14 - High Sierra might be the last OS to handle this step)
- Now your purchase is linked to your account, and you may reinstall those applications if they ever should get lost. So let's loose them: delete the
iWork09 folder from
Applications (or move it to the Thrash).
Go to the
purchased tab in
App Store. The applications are listed at the top of the purchased list. Upon selecting
install, you will be prompted if you want to install the last supported version for your OS release.
You now have
Numbers (v6.1) and
Keynote (v9.1) directly in your
Applications folder. (Version numbers applying to High Sierra as of January 2020)
If you have access to another Mac running a version of MacOS currently supported by the App Store (As of Jan 2020, that would be 10.14 (Mojave) or 10.15), you could associate your AppleID with an account on that Mac, download (and thus register) the Applications you want to install on your older Mac there, and then use the
Purchased tab on the older Mac to reinstall older versions of those apps on the older Mac.