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