How does someone on Catalina and with Keynote 10.2 update to the last supported version of Keynote for Catalina

As the current Keynote v11.2 now requires macOS 11 or later for installation, your only recourse to get this version is to upgrade your Mac to Big Sur or Monterey (when it is released). You would need a 2014 or later iMac to upgrade to Big Sur, and a Late-2015 (or newer) iMac for Monterey.

Apple does not provide full installer downloads of past Pages, Numbers, or Keynote versions (11.1) for Catalina. However, if you visit the Mac App Store, sign-in with your Apple ID, and press ⌘+0 (zero), this will take you to the past purchases panel, and you may be able to get an update from Keynote 10.2 to 11.1 there by clicking on a Keynote update button (if present). Otherwise, no solution, other than the first paragraph or using Keynote for iCloud, or Keynote in iOS/iPadOS 14.8/15.0.

This is a very unhelpful Apple policy, to not allow people with older MacOS versions to install Keynote that works on their system. My employer controls what version of MacOS I can install and I am stuck on Catalina for now. I have Keynote v11.1 on my computer (I have had it for many years), but a colleague who wants to view a Keynote document is not able to install it.

I am a fellow user. You can send feedback to the Keynote product team from its application menu because Apple product teams do not participate in these public user-supported communities. Apple has never provided user downloadable, full installers of past Pages, Numbers, or Keynote product versions, so you are facing established dogma.

Your colleague's options remain in the last sentence of my previous post.