Self-plug here.

I built a comment js plugin which hosts all data inside a git repo. https://github.com/est/req4cmt (as long as your git service accept http)

It runs a Cloudflare Worker for free. The data backup/migration is basically git clone & push

There's another twitter-replacement, also based on git. https://github.com/est/gitweets

Demo https://f.est.im/ it supports comments via git notes :D

$0.01/day ? They are all completely free thanks to Cloudflare Workers / Github Pages.