Is it though? My impression of HN is that debate isn't encouraged. Heck, discussion is basically discouraged. HN intentionally makes it more difficult to see if people responded to your comment to prevent people from constantly trying to defend their internet honor. However, back and forth discussion is important for debating ideas. HN guidelines specifically ask readers: "Please don't use Hacker News for political or ideological battle"

HN seems more geared towards "intellectual curiosity" and the discovery of new ideas. It doesn't seem tailor-made for rational debates, or specifically designed with debate in mind. I've yet to see a platform that attempts it (haven't looked into the posted site either).

Forums are for chronological discussion. New comments bury old comments. People talk past each other or in circles. A platform designed for rational debate puts a statement at the top and collects pro/con arguments under it. You win such a debate by curating a concise list of effective statements, not by talking longer than your opponent can.

Rational debate can happen in a discussion forum, but forums aren't really designed for it.

It'd be an interesting idea. I'm not sure about all of the features needed. I'd add that it'd be helpful to have a place to post sources, possibly with dedicated discussion. Both sides would have a chance to critique each others sources before the debate completes.

On Kialo the sources in the arguments are normally discussed in pros and cons underneath the argument they are in, or in the comments of a claim, sort of a meta discussion where folks discuss how to best formulate a claim. Some of their argument trees nest 10 levels deep.

I am actually not sure I would call Kialo debates, debates, they seem to me more like argument maps, coupled with Wikipedia. Also, they aren't antagonistic and there isn't really a winner.

I have gotten plenty of claims rejected because they were already in the debate.

I think the most important aspect of an ideal platform is that its users are not encouraged to write outrageous statements that then garner the most likes / upvotes, as is often the case on Twitter and Reddit.

At HN I am actually frequently surprised that things stay as civil as they do, though, as you write, they often go round and round and are a bit confusing for readers, which isn't HN's fault but just the nature of threaded commenting forums.