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 (see “A Note on the Nonbinary ‘They’: It’s Now in the Dictionary”). This use of “they” was recognized in the 17th edition of CMOS, published in 2017, and retained for the 18th edition, published in 2024 (see paragraph 5.52). So Chicago and Merriam-Webster are in sync on that.
Singular “they” can also be used as a generic pronoun referring to a person of unspecified gender, an established usage that nonetheless has long been considered informal. Starting with the 18th edition, CMOS acknowledges that this usage has become more common in formal prose and endorses its use in specific contexts (see CMOS 5.51 and 5.266).