You could use GitHub issues for posts and comments, fetched using the GitHub API. I did a proof of concept of this a long time ago [0]. Not sure if it still works.
Somewhat related, perhaps... Utterances (https://utteranc.es/), “A lightweight comments widget built on GitHub issues. Use GitHub issues for blog comments, wiki pages and more!”
Nooo I don’t want GitHub pages to get new features, it’s perfectly tiny right now