Lobste.rs is open source: https://github.com/lobsters/lobsters

But anything more complex than a basic server seems like overkill for a simple text forum.

Lobste.rs is a rails app - as much as ruby is not really my bag, I don't think there's anything about it that is particularly "not modern".

The implication from your post is that to be "modern" an application needs to be "serverless" - which frankly is preposterous.

I develop a HN/Reddit/Lobste.rs like internet forum [0]. It's Node/Postgres with a vanilla SSR front-end. It's not serverless.

[0] https://github.com/ferg1e/comment-castles