Other than one or two very minor issues, Keynote 7.3.1 is running fine here in all our Macs in Sierra.
Installing this Keynote update did not force you to upgrade to High Sierra, you chose to upgrade.
You can try resetting Keynote:
1 - uninstall Keynote; for this procedure to work, an application removal tool must be used, Appcleaner is free and works well for this. It can be downloaded from the developers website: Appcleaner
2 - Empty the Trash
restart the Mac: Apple Menu > Restart
3 - re-install Keynote from the Mac App Store
Finally, if you don't like High Sierra, you can re-install Sierra, details are here: How to reinstall Mac OS