The Wikipedia article seems to cover this quite well, but I'll say it here anyway: there are several kinds of singular "they". Using "they" to refer to a word

The Wikipedia article seems to cover this quite well, but I'll say it here anyway: there are several kinds of singular "they". Using "they" to refer to a word like "someone" or "anyone", as in "If everyone minded their own business", is totally normal and has been for a very long time. Using "they" to refer to a hypothetical or unknown person described by an ordinary noun, as in "The user must prove their identity", is common enough nowadays but I get the impression that it wasn't so common 50 or 100 years ago. Using "they" to refer to a known person, as in "My cousin lent me their copy", is very modern and likely to confuse some people.