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...)
That rarely happened with phpBB but I don't know about Discord. That fact makes me feel a little old.