Hi,
As a proof of principle I made this repository https://github.com/lcolladotor/vcontrol_keynote_test that shows how you can export a Keynote file to HTML, which creates a bunch of json files that you can then version control and whose diff will show the text changes you make between commits.
If you want the smallest repo, you can ignore the .key files and most of the files under the directory with the html files except for these .json files. There is an initialization step where you update your .gitignore and then for every update in your keynote file that you want to keep track of, you would need to re-export to HTML.
Best,
Leo