The answer seems to be, it depends.
As far as I can see, we are allowed to mention someone’s sex if it’s relevant to an objection you are making.
So if you are complaining about a man who claims he is a woman bringing a court case to say he must be allowed to play in the women’s section of a sports team or league, you are allowed to mention his sex.
If his lawyer is also a man who claims he is a woman who is watching the board and sees a post where his sex is not directly relevant to the role he is in, then your post is likely to be removed, presumably as such lawyers put pressure on Mumsnet, on principle in order to stop women talking in ways he doesn’t like.
You can mention his sex though, as long as you are making a point about how his sex affects something.
I should imagine Mumsnet is pretty sick of this situation, but still not certain enough of the law to want to become embroiled with such a case. There are certainly some very litigious male, woman identifying lawyers around.
So in short, we can mostly correctly sex people these days, but sometimes posts and occasionally whole threads are removed. Not sure whether we’re still kicked off after three strikes or not. I’m not aware of any recent bootings.