Using They as a Singular Pronoun

The Chicago Manual of Style is coming out with a new edition in September, and I’ve heard they’ve come up with a rule about using they, their, and them as singular pronouns. They say it’s acceptable in “informal” writing but not recommended for “formal” writing. Which begs the question … what constitutes formal vs. informal writing?

Does “informal” writing mean texting, e-mailing, sending a casual letter to a friend, whereas “formal” writing refers to a book? Maybe “informal” writing refers to self-published books, whereas “formal” writing would be a traditionally published book. Or maybe informal is someone who is only writing for family and friends, and formal applies to a book the author wants to actually sell?

In fiction, formal vs. informal writing could refer to certain characters’ dialogue and internal monologue. Teens or young adults, for example, might speak in a more informal way than well-educated adults.

I suspect most people will interpret this to refer to the book’s formal or informal style or voice. A theological treatise, for example, would be formal, whereas a children’s storybook would be informal. But most books fall somewhere in between those extremes.

As far as I can tell, CMOS isn’t going to include a specific definition of those terms. So we’re probably going to have some debates among writers and editors. I’d love to hear your thoughts on the subject!