I'd say that avoiding controversial topics is just something that happens when you struggle to maintain a good discussion. Sometimes you have to end a discussion because of how bad it is. But forbiding topics is throwing out the baby with the bathwater. It's admitting defeat.
When I see that a conversation is going south, I try to bring coolheadedness to it, or point out how it could be made better. I think that's the way to a good forum.
I think I see what you're getting at, so let me reassure you a bit, and correct for any miscommunication I may have caused:
> But forbiding topics is throwing out the baby with the bathwater. It's admitting defeat.
HN doesn't strictly forbid controversial topics. The guidelines[0] outline what is and isn't considered on-topic, provide heuristics for good behavior and pleas to avoid certain bad patterns. They don't say, "these topics will be automatically removed", or "these topics will get you insta-banned". They're just guidelines, a description of what this place should strive for. They work as a Schelling point[1] for individual participants.
So what happens in practice is, when you post a clear-cut intellectually stimulating hacker topic, it'll get upvoted. If it's boring, it'll get ignored. If you post a completely garbage submission, it'll get flagged faster than you can refresh the site. If a topic is on a fence, the balance of upvotes, flags and vouches will lean one way or the other. It's an organic system, but the direction it's leaning is strongly shaped by the guidelines - the one thing everyone knows everyone is expected to know :).
It's also easy to observe that whether or not a given topic stays on the front page or gets quickly flagged out of it isn't just a function of the topic and the source - but also the context (and I think sometimes the timezone). For example, political news aren't very prominent on HN, except when something really big happens, and HN briefly turns into a 24h news station.
And on the controversy itself, the general principle is - it's not the topic being controversial itself that's the problem. The problem is if the discussion around it decays into low-quality bickering. For some topics this is almost a law of nature, so people flag them on sight. But others only get flagged down if and when they evolve into actual flame war.
All in all, I think it's a decent system.
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[0] - https://news.ycombinator.com/newsguidelines.html
[1] - https://en.wikipedia.org/wiki/Focal_point_(game_theory)
Some topics (like politics) naturally induce more impulsive, emotional reactions than others (like innards of Erlang's BEAM runtime). The more you allow the former to dominate the latter, the worse average discourse gets, and the faster good people leave - it's a feedback loop that quickly grinds a community down to the lowest common denominator.
That's why stable, quality communities either expel (like HN) or contain (like plenty of forums limiting politics to only the Offtopic board) such topics. And it's not like it's a new invention - a lot (possibly most) societies have a long-standing custom that says not to bring up politics and religions in typical conversations between strangers (or at dinner with extended family).