Back in October of 2004, Geoff Pullum discussed a wall
inscription where they had a singular definite antecedent: "This
person is not ignorant. They are a prophet." Geoff wrote that
The pronoun form they is anaphorically linked in the discourse
to this person. Such use of forms of they with singular
antecedents is attested in English over hundreds of years, in
writers as significant as Chaucer, Shakespeare, Milton, Austen,
and Wilde. The people (like the perennially clueless Strunk and
White) who assert that such usage is "wrong" simply haven't done
their literary homework and don't deserve our attention. But
Geoff left out the single most compelling example. In the King
James Version, Deuteronomy 17: 2: If there be found among you,
within any of thy gates which the LORD thy God giveth thee, man
or woman, that hath wrought wickedness in the sight of the LORD
thy God, in transgressing his covenant, 3: And hath gone and
served other gods, and worshipped them, either the sun, or moon,
or any of the host of heave