There's not a lot of actual censorship of dangerous information "for the future of mankind". Or at least, I rate that as fairly unlikely, given that when the scientific groundwork for a breakthrough has been laid, multiple people usually invent it in parallel, close to each-other in time. Which means that unless you can get everyone who researches dangerous-level AI into LW, censoring on LW won't really help, it will just ensure that someone less scrupulous publishes first.
"Three may keep a secret, if two of them are dead."
Conspiracy is hard. If you don't have actual legal force backing you up, it's nearly impossible to keep information from spreading out of control -- and even legal force is by no means a sure thing. The existence of the Groom Lake air station, for example, was suspected for decades before publicly available satellite images made it pointless to keep up even the pretense of secrecy.
For an extragovernmental example, consider mystery religions. These aren't too uncommon: they're not as popular as...
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