I think warnings about information being actively harmful have been pretty rare, though.
There are very few, if any, societies without censorship.
But then there're rare one-offs, the ones that people have called basilisks, and with confirmed deaths or psychological injuries to their credit. These are clearly not in the same league.
I need examples, more than the present post ("hey, here's a rambling crackpot 2000-page suicide note") or, in the case of the LessWrong forbidden post, individuals with known mental disabilities (OCD) getting extremely upset. (I don't deny that they were upset, or that this is something to consider; I do deny it's enough reason for complete suppression.)
Would your criteria ban the song "Gloomy Sunday"?
It seems to me that starting with analysis at a distance is a necessary (but not necessarily sufficient) precaution in that handling.
It's catalogue of citable examples time, then.
Claims of real-life examples of the motif of harmful sensation are not rare at all. Substantiated ones are rather less common.
jimrandomh had other very apposite comments in private message which I've responded to. I don't think we deeply disagree on anything much to do with the issue of the necessity of learning to stare back at basilisks.
My various interweb browsings stumbled me upon a potential Cockatrice in written, philisophical form. I've thus far read through the first chapter, and it is less anti-rational than most philosophical writings.
I'm reading through it right now, and will provide my feedback when I'm done, likely as a front-page post.
Personally, I'm a Fatalist, with some sort of Weird Soldier Ethic, who plans to go out the same way that Hunter did (if the cops don't get me first), but I've got a bunch of nonsense to Write first. I figure that'll make me somewhat immune. That aside, I doubt it's a real cockatrice - or we would've heard about it before.
It is a strong exercise in Nihilism. So, with those cautions given, I offer it to you: an extensive suicide letter.
Tip of the hat to this guy.