Q. Now that Merriam-Webster has legitimized the singular “they,” where does CMOS stand on the subject?
A. First, please note that there are two uses for the singular “they,” generic and specific. In 2019 Merriam-Webster added a specific sense of singular “they” to refer to a person who does not identify with a gender-specific (or binary) pronoun (“A Note on the Nonbinary ‘They’: It’s Now in the Dictionary,” Merriam-Webster, September 19, 2019). This use of “they” was recognized in the 17th edition of CMOS, published in 2017 (see paragraph 5.48). So Chicago and Merriam-Webster are in sync on that.
Singular “they” is also used as a generic pronoun referring to a person of unspecified gender, an established usage that nonetheless has long been considered informal. As of the 17th edition, CMOS recognizes that such usage is gaining acceptance in formal writing but still advises avoiding it if possible—for example, by rewriting to use the plural (see CMOS 5.255). Generic singular “they” has been around for a long time, however, and most editors here at Chicago have no problem with such constructions as everyone should bring their favorite book to the event—where “their” refers back to the indefinite (and usually singular) pronoun “everyone.” And many of us have come to accept less firmly established usages such as each programmer worked in their preferred language. Like it or not, “they” has been displacing “he or she” and similar attempts to write around the English language’s lack of a dedicated gender-neutral singular pronoun for some time now. Stay tuned for further developments.