Although there were probably not any “non-binary” characters in Star Trek TOS, There were definitely some in Star Trek TNG. There was even an episode where the character played by Jonathan Frakes had an affair with an alien whose planet forbade HETEROSEXUAL relations.
Deep Space Nine had a character named “Dax” who was a male symbiote living in a very attractive female body. The captain even had a nickname for her: “Old Man”.
Star Trek Voyager had a chef/moral officer who pranced a bit, but it was never overt.
No way is anything today going to be a “FIRST” on Star Trek. It’s all been done.
#49 When I figured out what Dax was a guy it was yuck!
Plus the Klingon Worf married him or it.