Yeah - one thing I dislike about Reddit - it has eaten up all of the small forums and killed them. On one hand Reddit can be great but on the other hand sometimes you just want to discuss some niche stuff or hobby - and too much other crap will spillover from other subreddits sometimes to make that as useful
In what way does crap spillover from other subreddits? Are you talking about the new Reddit GUI (I use the old one) or mobile app (I use RedReader) spamming posts from other subreddits, or a more cultural aspect of site-wide conflicts?
It happens if a niche reddit has a popular post and makes it near the front page. Or, someone makes a post that gets the attention of one of the brigading subs(even if mods remove post as soon as they see it, it might be too late). Usually the sub will have an influx of low quality posts/shitposts for a while after that.
Also, not subreddit-subreddit, but multiplex times, posts of mine got reported to admins for hate speech because I said "retarder" in a mechanical focused subreddit, and I suppose they don't care to take the time to realize it's a perfectly fine piece of technology and not an ableist slur before sending out threats of banning you forever from the site.
Reddit has active communities of people who just repost other subreddits to laugh at, get outraged at, or bully. In theory the bullying and brigading is supposed to be moderated against but weakly moderated niche subreddits can sometimes find themselves under attack.
That rarely happened with phpBB but I don't know about Discord. That fact makes me feel a little old.
Sites for pointing at people and mocking them for being different have been with us much longer than Reddit. SomethingAwful. 4chan. LJDrama. alt.fan.warlord. The current main site in this space is Kiwifarms and I sure won't be sad if Cloudflare finally decides to stop protecting them from the DDOS consequences of their actions.
Bulletin boards with any kind of roadbump to registration could make it a bit harder for a bunch of atrocity tourists to start showing up and mocking someone directly but you sure couldn't stop them from screenshotting you and kicking you around on their board.
(okay alt.fan.warlord is maybe a bit of a stretch given that it was generally confined to only making fun of people's signature blocks, but...)