I like warpus and Marla_Singer's points in particular. I don't use Reddit much, but I see the same differences in likelihood of long threads being toxic among the sub-forums here. It occasionally happens in the Civ forums, but quote wars and subsequent toxicity are more common in this sub-forum, which I see as being due to the subject material being more likely to result in strong reactions. The graph in the article primarily uses shades of brown and gray with the occasional blue to differentiate topics, so it's a bit hard to read the trends per-topic, but it can be seen that not all topics have the same proclivities.
I'm not a fan of the concept of expiry dates for threads, especially the Civ forums, as IMO the value derived from being able to bump a thread exceeds the downsides from toxicity due to thread length in those forums. I could see it being selectively useful in Off-Topic if threads have a pattern of becoming toxic when bumped, although, then, what's to say a new thread won't be started that picks up from where the old one leaves off?
I only took a peek at it, but my question is: is that better or worse than other forms of communication? For all I know, we might be better than cable TV or newspaper editorials.
At least for my in-person conversations, or direct instant messages, I don't see a trend of length -> toxicity. Go out to have a beer or two with friends, it doesn't become more toxic over time (and if it does, maybe that says something about your friends). Well, I suppose I have seen work environments where debates devolve into arguments that become more toxic over time, and those are not the healthy work environments. So, it
can happen in person, but it's not considered healthy when it does.
Newspaper editorials have implicit cooldown times and moderation. Sure, you could write a heated letter to the editor, but they have no obligation to publish it.
Cable TV, there are aggressive hosts who tend to cause the temperature to go up the longer their subjects are on-air. I don't enjoy watching them, and have wondered why people agree to go on those shows. Like, did they not watch a couple episodes first to get a feel for what to expect, and realize it may do more harm than good? To use the CNN circa 2010 example, I prefer Anderson Cooper to Nancy Grace.
Then I can listen to NPR on the way home, and it's very civil and informative, even if it's an hour-long show. So, it can once again depend on the venue.