Earlier quoted context omitted.
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?
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.
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...)