People are still fighting the seemingly inevitable adoption of they as a gender-neutral singular pronoun. One stalwart in this battle is The National Review, which claims:
English does have a gender-neutral third-person singular personal pronoun – it’s “he.” Per the dictionary of record, Webster’s Second International Unabridged, the primary definition of the pronoun “he” is “the man or male being previously designated.” The meaning of “male being” is self-evident, but the meaning of “man” has been forgotten by many badly educated people. The first definition Webster’s gives for “man” is “a member of the human race.” Webster’s gives a quote from Hume by way of illustration, “All men, both male and female.”1
Appealing to the 18th century for justification against they might seem unfair—but it's not. It's just wrong.
The Oxford English Dictionary traces the singular they at least as far back as a poem called William the Werewolf, written in 1375. Many writers, including Shakespeare, relied on the singular they for centuries.
In the 1600s, the singular informal you replaced thou, echoing the transformation of they from strictly plural. If The National Review had been around at that time, they might have railed against this grammatical evolution as well.2
Grammarians of the 1700s called the singular they an error, despite broad acceptance of the singular you. By the middle of the century, the fate of they was sealed. Anne Fisher, in her 1745 work A New Grammar, declared he the universal singular pronoun. The gender-neutral he was canonized under the Interpretation Act 1850, which the British Parliament passed to simplify the language used in statutes.
The 1800s saw a search for alternative pronouns. Samuel Taylor Coleridge suggested it and which. In 1864, a writer known as J. W. L. invented ze. By the end of the century, there were en, le, hi, ir, hizer, ons, e, ith, hiser, ip, thon, and many more. Thon was coined in 1884 by Charles C. Converse, composer of What a Friend We Have in Jesus, and appeared in Webster's dictionary as late as 1934. In 1930, Winnie the Pooh creator A. A. Milne suggested heesh.
None of the invented pronouns took hold, and with the grammatical prohibition of they, the gender-neutral use of he ruled the day for much of the 20th century. Although there were debates about gendered language, style guides and grammarians alike recommended against the singular they past the turn of the millennium. In 2008, in a post called "The epicene pronoun," Grammarphobia responded to a question about New York Governor David Paterson's use of the singular they:
The singular “they” or “them” or “their” has been considered wrong for a couple of centuries, and it’s still a no-no in formal English. But the governor, whose grammatical relationships have come into question before on the blog, is one of millions who adulterate the language this way.
Gradually, the tide has started to turn. Nine years later, in 2017, Grammarphobia updated their position on they in a post called "50 shades of they," stating:
“They” is a legitimate way of referring back to an unknown person or persons, neither singular nor plural, masculine nor feminine.
It is not surprising that style guides are slower to change. They recommend using they "in limited cases" (Associated Press Stylebook), trying to "write around the problem," (The Washington Post), or limiting the singular they to "speech and informal writing" (Chicago Manual of Style).
It seems clear, regardless of what other pronouns might emerge, that the singular they is back after 250 years of prohibition. If you know who you're talking about3—and their4 pronouns5—you should use the specific words that apply. Try not to rely on they when you know someone prefers zey/zem, for example. But if you are referring to persons unknown, or persons with pronouns unknown, they is suited to the job.
It's worth reading the whole article, if only for the shaky writing. For example:
Surely the American Dialect Society is aware of this. Certainly, they out [sic] to say so.
Oh, wait, they did:
English lost precision when “you” replaced “thou.” Ideally we’d still use “thou” as the familiar for an individual.
Yes, yes, about whom you're talking if you prefer.
I don’t know about whom I’m talking.
A few years ago, despite being cisgender, I considered adopting they/them as my pronouns, partly in recognition of they as a legitimate form and partly as way to help denormalize traditionally gendered pronouns. In the end, I thought doing so might come across as disingenuous or appropriative, and I didn't want to saddle myself with explaining over and over again what I'd meant to do.