Apple only provides the current release of Keynote in the Mac App Store and presently that is v10.1 which requires Mojave 10.14 or Catalina 10.15. Otherwise, Apple does not provide user downloadable, full versions of older Keynote installers. However…
If you have had Keynote installed previously, under the same Apple ID that you are currently using in High Sierra, you might be able to get it installed via a different means. Launch the Mac App Store, and sign-in with your Apple ID. Press the option key while clicking on the Purchase tab. If you see an active install/update button with Keynote, then click it.
The Mac App Store will tell you that it cannot install the current version of Keynote, but it may offer you a consolation, by offering to update/install the latest Keynote v9.1 for High Sierra. If it doesn't then you are done.