I think it may be both. Engagement algorithms are definitely part of the problem. Agree it was far more fun when it was random / organic interactions.

However, I also think it's kind of like a party that starts out like a book club, it gets more interesting, and then louder and more obnoxious folks hear about this, and they keep showing up.

By the end it's a completely different vibe, and the original folks are long gone. Have experienced it numerous times over the long years, before the sorting and engagement algorithms joined the fray.

I know this comes off as kind of hipsterish. But, most obnoxious people don't realize they are obnoxious. And confronting them seldom does anything but escalate the situation. So leaving is the mature choice. Therefore... mature folks leave, and the forum's relative aggregate immaturity goes up.

One way to fight it is with very strict moderation, and I have seen that work. But it's labor-intensive and requires moderators who are highly dedicated and fair, and don't "power trip". I'm not a huge fan of that approach overall. But in the right context (like academic discussions) it can be pretty good.