Trying to put words to my own intuitions on the matter, I would stipulate a modified 3:

It may be unsafe (in terms of image/status/etc - I would certainly expect and hope not physically) to express certain views, particularly those sufficiently far from both societal mainstream and LW mainstream, and particularly those that touch too heavily on mind-killing topics.

It is reasonably within norms to acknowledge this, particularly with an eye to reducing its effect.

What is decidedly a violation of norms, I think, is to do so in a self-serving manner.

"Norms forbid honest discussion of my pet issue X, therefor X" is obviously flawed.

"Norms forbid discussion of my pet issue X, and I have strong evidence for X but can't share it because of those norms, so just trust me that X" amounts to the same thing, in terms of what kinds of discussions are possible. It is also, to some degree, inconsistent - it is unlikely that we forbid evidence for a proposition while allowing discussion otherwise implying/assuming it.