Because this often ends up with people talking past each other, never until a few years ago did anyone use singular they for a known, specific person. Shakespeare used it for unknown or nonspecific people.

It was also used in the King James Bible, published in 1611.

I'd be shocked if that were universal over that time, given that even formal language has undergone many changes in attitude. Over hundreds of years, I bet that in many times and places it has not considered it a problem, particularly given its use in the King James Bible.