'They' is plural, never use it for one person

What actually happened

The historical record is unambiguous. Singular 'they' appears in Chaucer, in the 1611 King James Bible, in Shakespeare, in Jane Austen, and in Dickens, and it was unremarkable for four hundred years. The objection was manufactured in the eighteenth century, when grammarians pushed generic 'he' on Latin-derived logic, and Britain's 1850 Act of Parliament formalized masculine-as-generic in legal drafting.

Generic 'he' was the real innovation, and it lost. It reads as excluding half the population, and 'he or she' is clumsy and cannot be repeated more than once in a paragraph without collapsing. English had a solution the whole time and had been using it continuously in speech.

The 2017 dates mark the formal turn: AP and Chicago in the same year, both permitting singular 'they' for indefinite reference and for people who use it. Merriam-Webster's 2019 Word of the Year followed a 313 percent rise in lookups, and the entry for the nonbinary sense had been added that September. The remaining discomfort is generational rather than grammatical, and it belongs to a rule that was never native to the language.