Is there an older version of Keynote or Pages that I can download to a macOS, version High Sierra 10.13.6?

Not directly, no.

If you can get access to your own account on a computer with the required minimum macOS version for the current version you can download the new version of the app to it. Now go to your old computer and search for the application in the Mac App Store application. Try downloading it. It may offer an older version if one is available. If this is an app that uses a service there is no guarantee the old version will actually work with the service but I have found some old versions are available and provide partial functionality.