The language, yes[0], but AFAIK the maintainers don't take pull requests.
Arc has a public fork called Anarki[1], which is built on Racket[2]. The Anarki version of the forum differs from the Arc forum, which differs from HN's own custom instance, which is closed because of various YCombinator business reasons.
And to be a little more specific (because I really like Arc), the releases are very rare, and minor at this point. I would be surprised if the Arc that was running HN was the latest released version of Arc, the news library notwithstanding.