I moderated a cycling community (r/mtb) that grew to about a quarter million subscribers and I had a really difficult time keeping the low effort content at bay.
When communities grow to a certain size they lose their niche audience, and lowest common denominator posts bubble to the top and drown out quality content.
That said, I like PinkBike.com for MTB stuff and Hacker News for tech stuff. Those are really the only two forums where I actually leave comments these days.
It isn't just the quality of the posts that degraded though. r/MTB for example hasn't gotten as spammy as most other subs. But the quality of the community has gone to shit, and thats almost worse because that was the main value it originally had. There really good gear discussions and technique help is gone. Most posts get 1-4 comments, almost all of which are "where this at?" or "what bike u hav?" that there is no community anymore. No discussions, which to me was the real value of the sub.
There is one subreddit that I don't feel comfortable mentioning because it covers a deeply personal and sensitive topic. But I joined that community back when it had 5k subs (it now has 300k). That community saved my life in many ways and provided therapy and healing by discussing a sensitive topic and help with recovery for people who were going through a similar traumatic life experience that I was going through at the time. It was an invaluable support group. Many of us knew eachother (by anonymous usernames of course), and helped eachother cope through very tough life experiences.I even participated in a few IRL Meetups. People would write research and high-quality content on there that rivaled academic content available at the time, several actually went on to do PhD research in this previously-unresearched niche. It was such an exciting time and such a healing and powerful community.
That community is 300k people now and a complete disaster. Way too many posts, most of which are off-topic. The remainder of the posts are non-constructive rants (which were previously banned). To make matters worse, there was a period of time where the community became unsafe and a mod from an opposing group had become a mod of our support group and was doxxing people in a way that ruined a few people's lives (I was luckily mostly no longer an active participant by this time, and mostly just lurking and providing occasional guidance). The community is now very rage-filled and is nothing like the quality support group/community that I was able to participate in.
Reddit is all about the communities. But it has become so commercialized and mainstream now that it has lost its original value. I don't want to scroll through a feed of memes and low quality shit posts or reposts. I want discussions and q+a forums with people who are passionate about topics, hobbies, life experiences, and shared interests. I want real discussions, not another algorithm-driven social media network spamming garbage at my eyes.
The Digg migration was the first step towards Reddit's Eternal September, but Covid lockdown was the final nail in the coffin.
Reddit is the complete opposite these days. Censorship (often arbitrary) and banning are out of control. In extreme cases they ban users for making a joke, I've seen that more than once. So when you check old threads and look at the usernames, you'll find quite a few of them don't exist anymore because they were banned. On top of that, each subreddit has their own moderators/dictators that quite literally simply remove anything they don't like.
My theory is that a significant portion of persistent trolling is caused by that. Users are or feel treated unfairly, they're bitter about it and then some of them come back with an alt account and disrupt the community. Then there's the animosity between some subreddits too, like you hinted at. A good portion of reddit these days is drama. Since it drives engagement one way or another, the admins are unlikely to crack down on it.
If the comments under posts become too active, it is downrated and disappears from the front page which reduces the rates of people commenting on it.
https://news.ycombinator.com/item?id=16020089
You'll also note things like "when the comment is too deep the reply link disappears" which creates a bit more friction for carrying on what would likely be unproductive threads.
Things around controversial topics also get downranked (whereas on Reddit hot topics move to the top of the list).
There's a lot of automated tooling in HN that reduces the amount of manual moderation that is needed by making it harder to have posts that contain things that need moderation.