[pron]

That doesn't limit in any way the expression of the belief that there are only two genders. Feel free to run a Twitter search and confirm that for yourself (I just did). I have to say I am genuinely curious to find a conservative belief that I wouldn't find thousands of (unironic/uncritical) hits for in a matter of seconds.

[sneed-oil]

It does, you can't tell a non-binary person that they're a man or a woman, if your tweet is reported it will be deleted. If you can find any examples of this they simply haven't been reported or reviewed yet.

[pron]

Intentional misgendering of an individual could be sexual harassment; it is certainly not a form of "conservative speech" regardless of beliefs. However, the belief that there only two genders is not only not banned by Twitter but is, in fact, widely disseminated.