> If my forum has a narrow scope (say, 4×4 offroading), and I delete a post that’s obviously by a human but is seriously off‐topic (say, U.S. politics), does that make me legally liable for every single post I don’t delete?
No.
From the court of appeals [1], "We reach this conclusion specifically because
TikTok’s promotion of a Blackout Challenge video on Nylah’s
FYP was not contingent upon any specific user input. Had
Nylah viewed a Blackout Challenge video through TikTok’s
search function, rather than through her FYP, then TikTok may
be viewed more like a repository of third-party content than an
affirmative promoter of such content."
So, given (an assumption) that users on your forum choose some kind of "4x4 Topic" they're intending to navigate a repository of third-party content. If you curate that repository it's still a collection of third-party content and not your own speech.
Now, if you were to have a landing page that showed "featured content" then that seems like you could get into trouble. Although one wonders what the difference is between navigating to a "4x4 Topic" or "Featured Content" since it's both a user-action.
[1]: https://fingfx.thomsonreuters.com/gfx/legaldocs/mopaqabzypa/...