I noticed that as well. I think it may have been manual in this case, going by what I've observed in the past. YC has some idea of what it doesn't want on the f

I noticed that as well. I think it may have been manual in this case, going by what I've observed in the past. YC has some idea of what it doesn't want on the front page of its forum, and has paid staff who remove it. There are algorithms promoting and demoting content too, but afaik they don't have this effect. They either flag posts e.g. if detected as spam (which isn't the case here), or they penalize them, which might push them down to the second or third page, but not remove them from being ranked entirely. A page remaining unflagged but removed from ranking is more unusual, and I think in the past has been manual intervention.