I (wrongly) extrapolated from the handful of trans people I know who focus on passing. Either way, without personal conversations about gender identity, how would it come up?
I think this is the winner right here.
Unless the trans character is a main character... and even IF they are... why would we even have this information? At what point does their trans identity become relevant to anything going on? I understand how same sex couples would come up... people have relationships, so it's quite easy to represent that by showing their relationship. But trans? The only way we would ever know is if the work/character goes out of its way to tell us.
I have a story of my own with a trans woman character in it. But... the reader would probably never actually know this, because I just write her as a woman. At no point in the story does she feel the need to say "Oh, by the way, i'm TRANS!"... it's just not relevant to anything. If there was a reason for it be relevant, sure, but as of now I haven't encountered anything.
There is also a part of me that wonder this... since gender is a social construct, and we know society has changed quite a bit by the 23rd/24th century. Is it then outlandish to think that with the changing society, the amount of people identifying as trans may just be less than it is in 2024?