It’s true that style guides — which are often prescriptivist (“Do this”) rather than descriptivist (“This is what’s done”) — argue against using it, at best warning that writers who employ it may be considered to be in error. The Chicago Manual of Style, for example, advises, “While [shouldn’t that be although?] this usage is accepted in casual contexts, it is still considered ungrammatical in formal writing.”
Many literate people who use the singular they in speech hesitate to do so in writing because of this prejudice. As a result, too, there is a lingering resistance among many editors to allow it.
However, the singular they is widely accepted in written British English, and it is well documented in the works of many great writers, including Auden, Austen, Byron, Chaucer, Dickens, Eliot, Shakespeare, Shaw, Thackeray, and Trollope. It was the singular pronoun of choice in English for hundreds of years before, in 1745, an otherwise-reasonable grammarian named Anne Fisher — yes, a woman — became possibly the first person to champion he as the universal pronoun of choice.
According to Merriam-Webster’s Collegiate Dictionary, “The use of they, their, them, and themselves as pronouns of indefinite gender and indefinite number is well established in speech and writing, even in literary and formal contexts.” Meanwhile, R.W. Burchfield, editor of The New Fowler’s Modern English Usage, and Bryan A. Garner, in Garner’s Modern American Usage, predict the inevitable dominance of the singular they.
I am flummoxed by the controversy over it and by the resistance of many people to accept it. Singular they has long been used in literature and in conversation, and though it still has an informal taint, it seems to me absurd to resist adopting it when it satisfies an aching need.
ATTENTION WORLD THEY CAST WONDER WOMAN AND THEY CASTED AN ISRAELI ACTRESS NOT SOME PRETTY WHITE WOMAN LIKE HOLY SHIT THAT’S HUGE.