How to download Keynote

If you upgrade to Monterey, you will be able to directly download Pages/Numbers/Keynote versions 11.2 (presently) from the Mac App Store, and never bother with the Purchased panel.

Keynote v11.2 requires that you have macOS Big Sur or later installed. The last compatible version of Keynote for Catalina is v11.1. If you already have Keynote v11.1, then read no further as that is the last available version for Catalina. Otherwise…

Launch the Mac App Store signed in with the same Apple ID as you have used in the past to install Keynote. Press cmd+0 (zero) to open the Purchased panel, and click the Update button adjacent to Keynote. This will first present an obvious dialog. Accept that dialog (do not press Cancel), and a second dialog will then appear offering to install the last compatible version of Keynote for Catalina.

It used to be that when a new Mac from Apple arrived with preinstalled versions of Pages, Numbers, and Keynote, that one would have to first visit the Purchased panel that I mentioned and accept those installations by clicking on the application before using or updating it.

The removal of Keynote from your Mac should not have removed its presence in the Purchase panel of the Mac App Store, because there would be installation history on your Mac. Without Keynote's presence in the Mac App Store Purchased panel, you will have no means of obtaining it for Catalina because Apple does not provide downloadable full installers for past versions of it.