They

Since English already has an example of a pronoun that is used both as a singular and a plural and used that way in grammar so proper even the snootiest grammargendarme prescribes it (I’m talking about you, of course), I don’t see why we can’t allow that sort of use for another pronoun. Everybody knows that using he or she in order to make sure there is agreement in a sentence feels tiresome and awkward, especially when speaking. Yet it is conventionally considered the best alternative when looking to de-gender the generic. “Everybody loves to kiss his or her lover!” Please. “Everybody loves to kiss their lovers!” Everybody is supposed to be a singular, according to the grammar logic crowd. But everybody knows that in real life, outside the textbooks, it’s a plural. “Everybody get out your textbook.” That’s grammatically acceptable, isn’t it? “I want everybody to get out their textbook.” Not correct! No? Pooh on that. “I asked for a wheat-back penny. Suddenly everybody was looking through their change.” I’m grammar-sensitive enough to wonder if sentences where a “their” sounds natural in speech could or should be rephrased when written. “Suddenly you could hear the jingle of change as every hand dug through their stash.” Oops. Did it again. “Suddenly everyone was looking through his or her coins.” “Suddenly all the kids were digging through their change.” Functionally, “all the kids” and “everybody” are equivalents. We treat them that way in speech. I think we ought to treat them that way in writing. English has a very formal neuter - one. “One ought to know one’s mind.” It sounds a bit off, a bit British, even. And using one that way has its problems, too. “One was standing on the platform when the train arrived.” One was? One what?

In Our Magnificent Bastard Tongue John McWhorter looks to see if there’s any their there:

Take the idea that it is wrong to say If a student comes before I get there, they can slip their test under my office door, because student is singular and they ‘is plural.’ Linguists traditionally observe that esteemed writers have been using they as a gender-neutral pronoun for almost a thousand years. As far back as the 1400s, in the Sir Amadace story, one finds the likes of Iche mon in thayre degree (‘Each man in their degree’). … Shakespeare is not assumed to have been in his cups when he wrote in The Comedy of Errors, ‘There’s not a man I meet but doth salute me / As I were their well-acquainted friend’ … Later, Thackeray in Vanity Fair tosses off ‘A person can’t help their birth.”

McWhorter has been asked, if this use of they/their/them is so appropriate why don’t linguists use it themselves? It’s the copy editors, he says. “[E]ven linguists have to submit to their publishers’ copy editors’ insistence on expunging it … At best I can wangle an exception and get in a singular they or their once or twice a book.”

It may still be that books are copyedited by human beings, but most of the writing on the internet clearly isn’t. No copy editor touched this post, for instance. Other than me, and I don’t claim the title.

source: Our Magnificent Bastard Tongue: the untold history of English by John McWhorter