Old version (macOS Sierra) for Pages, Keynote and Numbers, iWork
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 Pages (v8.1), Numbers (v6.1) and Keynote (v9.1) directly in your Applications folder. (Version numbers applying to High Sierra as of January 2020)


Alternative route
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.

Sources:


2014 Redmondpie article
2014 Cult of Mac article
2018 osxdaily article "Where to Download Old Mac OS Software"
MacWorld article about missing purchases tab in Mojave App Store