CyberGene
Johnstaf They/them has been used for centuries to refer to a single person without specifying their gender. I've always used it, and always heard it used.
I'm the last person on this forum to discuss the English language 🤣 But I think there are some nuances here. I believe it may have been used in the past for being polite (or when you don't know the actual sex of a person because you exchange emails. For instance, since I work for a big corporation with many offices, it's hard for me to determine the sex of a person from only his/her name and we've even made some funny mistakes in Bulgaria because in our language the feminine and "third" gender of words, sorry I don't know the grammar terms, always end with a proper suffix which determines the gender and thus one can unconsciously decide that a name is of a female.)
And in the last 10 years or so the accent of "they" is not because you don't know the sex of the other person, but rather to allow for "them" to assume a sex that is different than the apparent one. Or in other words it includes the possibility of the other person being transgender. At least that's how I understand it.