For example, spamming advertisements in comment sections is completely legal speech. So would be typing in gibberish and hitting enter a thousand times. But both of these things would ruin the point of the forum.
Even if I compare Hacker News to Reddit, the former is consistently high quality while the later is 95% garbage in my opinion. Why? Probably because Hacker News is far more highly curated.
At the same time, I feel like YouTube has crossed the line with idea suppression. YouTube gives the impression that it's an open non-biased platform that links you to content you're interested in. But there are several egregious examples where popular videos with near-mainstream "conservative" viewpoints are suppressed into oblivion (e.g. appears on the 12th page of results even when you search for the video title verbatim and it has millions of more views than every other "relevant" result).
But, just because I can give examples of things which cross the line (suppressing popular conservative videos) vs things that don't (suppressing random gibberish, suppressed bot-created videos)... I don't think I could clearly articulate any rules to say exactly where that line is in a way that is scalable. YouTube created YouTube... I might just have to defer to their moderation policies while hoping another competitor comes along to challenge them.