And the only answer from eesmith cites a very old implementation, I'm wondering what's being used nowadays in the HN code base
It lists the tech stack as Rails, React, Postgres.
https://www.ycombinator.com/careers?ashby_jid=ef00c8d1-76e7-...
If so, probably started with a database that you would commonly pair with a Lisp language
And the filesystem.