citation needed, I've seen many reddit threads degenerate into name-calling, logical fallacies, political posturing, and what have you. If they don't, my guess is it's a very focused subreddit that doesn't attract much general interest, or the mods are very on top of things.
You can choose not to engage with that branch and instead start your own.
The only benefit for a forum is that it "bumps" a topic when you participate in it, so you can keep a topic alive. But that isn't too difficult for a threaded style forum to replicate. That just wasn't how Reddit wanted topics to work (nor HN, in our case). Reddit never even implemented a way to subscribe to a topic despite a 3rd party extension having a feature for over a decade.
Maybe it's better to say "it's easier to mute de-railings" then say it is prevented. one or two clicks compared to a dozen page loads and scrollings.