Ask HN: What HN-like forum software is substack using?

One of the founders of Substack here.

It's built in house. Thank you for the kind words. I'm not sure I'd call it a clone of HN, but that's definitely one of our influences :)

Check their jobs page: "We use: Node, Express, React, Postgres, Heroku"

What I can see is not that different from Disqus that has a free alternative called Isso https://posativ.org/isso/

My money is on in-house.

Express based webapp on Heroku. If it's a plugin, I would start looking in NPM

It looks like it's built in house.