I genuinely don't understand why you would subscribe to a community about 4chan greentexts and not expect to see shocking language. Its like going to the beach and being upset that theres sand.
It's why there's a big warning on the sidebar that says "Beware: sometimes Anon says bad words or things that are unhealthy. We're here to laugh at Anon, not agree or get angry."
Unlike reddit, there’s no “default subs” on Lemmy. It’s either “all” or “subscribed”. All literally means all communities, so yes, objectionable stuff will show up in your feed. The solution is to block them, then they won’t show in All any more.