The current source of Hacker News is proprietary, because this forum is attached to a billion-dollar startup incubator and there's apparently real money to be had for wantrepreneurs trying to game the algorithm, and plenty of "business logic" they would have to remove and whatnot.

The current source of Arc Lisp is at https://arclanguage.org. It isn't open source in that there is any way to contribute or make pull requests that I'm aware of (I may simply be too much of a pleb to know,) rather now and then new versions simply descend from the Lisp gods and are posted. So it's more 'source available.'

The current public fork of Arc Lisp is Anarki at https://github.com/arclanguage/anarki. It has deviated a great deal from Arc and its version of the forum is not in any way in feature parity with Hacker News. But anyone is welcome to make a PR and contribute.

The Arc language forum is at https://arclanguage.org/forum.