The prohibition on singular "they" always struck me as the same sort of pointless and incorrect overcorrection as the ban on split infinitives (I dare to boldly split them) and terminal prepositions (a sort of nonsense I won't put up with).
Proponents of singular they have long argued that the prohibition makes no sense. Not only is it natural, it has been used in English for centuries. It’s in the King James Bible. Authors like Chaucer, Shakespeare, Swift, Austen, Thackeray, and Shaw used it. Before the production of school textbooks for grammar in the 19th century, no one complained about it or even noticed it. Avoiding it is awkward or necessitates sexist language.
Now, in the most recent update to The Washington Post style guide, singular they has been given official approval. Post copy editor Bill Walsh explains that he personally accepted singular they many years ago, but had stopped short of allowing it in the paper. He finally decided to endorse it in house style after coming to the conclusion that it is “the only sensible solution to English’s lack of a gender-neutral third-person singular personal pronoun.”
Other institutions are sure to follow suit.... The news of the acceptance of singular they may cause a little stir, but nobody will notice the change in action, as Walsh says, “I suspect that the singular they will go largely unnoticed even by those who oppose it on principle. We’ve used it before, if inadvertently, and I’ve never heard a complaint.”