Getting keynote on an old mac

You can get the version (9.1) of Keynote that was last compatible with macOS High Sierra only if you had installed Keynote previously from the Mac App Store (since Oct 2013) and with the identical Apple ID. Apple does not otherwise keep past full installers of their applications available for user download.

If you meet the first paragraph's criteria, then launch the Mac App Store and sign in with that same Apple ID. Press and hold the option key while you click the Purchases toolbar item. If Keynote appears on that Purchases panel with an Update/Install button, click that. You may receive one or more dialogs, neither of which should you click Cancel. The final dialog will install Keynote v9.1 into your /Applications folder.