If you have previously installed Keynote using the same Apple ID since Fall 2013, then you can get the last compatible version for Big Sur. If the first sentence is false, then you cannot get Keynote for Big Sur.
Assuming true, launch the Mac App Store on Big Sur and be signed in with the same Apple ID above. Press cmd+0 (zero) to open the Purchases panel and you should see a blue cloud download symbol adjacent to Keynote. Click that and a dialog should appear offering to install the last compatible version of Keynote for Big Sur.