John McWhorter finds that it was commonly used by various literary greats:
So why is the notion that they or their is plural as unquestionable as the law of gravity? It traces back not to some self-standing “canon” or scientific principle, but to an English grammarian who, as far as anybody can tell, just made it up in the 1740s that he should refer to both boys and girls. (For what it’s worth, this grammarian was a woman, named Ann Fisher.) The notion was then passed down through the centuries, such that Strunk & White taught us not to use it. Apparently William Strunk had a certain classroom charisma, but are we really to heed his aesthetic sensibilities over Shakespeare’s or Thackeray’s?