It's already fine for singular, when you don't know someone's gender.

e.g. "I just called my doctor." "What did they say?" if the 2nd speaker doesn't know the 1st speaker's doctor.

That's why it's also okay for anonymizing and non-binary genders.

What I just said it's that's what most people do. I clarified that this is in spite of "they" typically referring to multiple people. It's helpful for non and new English speakers if you explain these colloquialisms.