So says Merriam-Webster.
Here’s Benjamin Dreyer celebrating the decision in the Washington Post. As long as I’ve been around, people have been using “they” to refer to a person whose gender was unknown or irrelevant — and then, perhaps, feeling guilty about it. Or, we tried to recast the sentence to make the person in question plural, or we held our noses and used the awkward “he or she.” That dodge has become more than merely awkward as an increasing number of people reject the binary he/she as their pronouns of choice.
Merriam-Webster is fundamentally descriptivist, so this accolade doesn’t mean they are saying “they” must be used in this way. On the other hand, other organizations, like the American Psychological Association, have endorsed the usage. But what’s most important is that my cold-eyed editors reached the same conclusion earlier this year. There is, of course, no higher authority, This means that you can feel confident that you are doing the right thing when you use the singular “they.”
Go ahead. You know you want to.
Richard Bowker 
Never mind you’re editors. What would your english teacher(s) in high school say?
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C’mon, Joe, it’s “your” not “you’re.” My editors wouldn’t let that one go. Anyway, high school is a long way in the rear-view mirror now. Times change.
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Actually I don’t want to. More importantly, does this mean we’re going to have to get used to “they is”? What do your editors have to say about that?
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Well, that second sentence is a pretty good example of slippery-slope-ism. “If a guy can marry a guy, next thing you know he’ll be able to marry his dog.” That sort of thing. Merriam-Webster is descriptivist, and they’re describing what they’re seeing. My editors are prescriptivist, at least as far as the words produced by our company are concerned. And they’re saying to our writers: Don’t tie yourself in knots to avoid the word “they” in constructions where it’s common. M-W isn’t seeing “they is” in standard American English, so it’s not a thing as far as they’re concerned. And our writers aren’t writing it, so it’s not a thing for my editors to worry about.
Your phrase “More importantly” interests me. Google “Is more importantly grammatical?” and see what you get. Or just read this article from M-W: https://www.merriam-webster.com/words-at-play/we-know-youre-concerned-about-important-and-importantly.
At one time many people thought using “more importantly” in the sense you’re using it was a grammatical mortal sin. Several decades ago the president of the organization I was working for ordered me to destroy the print run of a brief article I’d written because I used the phrase “Most importantly” when he believed I should have written “Most important.” Stung, I looked it up, and he seemed to be correct. (It was good to work at a place where people cared about stuff like that, though.) But time passes and language changes, and it looks to me now like “more/most importantly” is winning. Alas! Chaos looms. The center cannot hold. What’s next? Split infinitives? Sentences beginning with a conjunction? “They” used in a singular sense? The horror!
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Do you still have any copies from that print run? 😁
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Long gone. Worth a lot now, of course!
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