I'm an English professor who's been publishing since the mid-nineties or so, and I don't recall ever seeing publisher's guidelines that didn't insist on gender-neutral language when possible (I believe it's been in the MLA and Chicago guidelines for a long time as well).

I've also never had an editor object to my using "they" in this way (usually to avoid repetitious use of "he or she" or the rather clumsy -- to me, any way -- "he/she"). Sometimes I alternate, which is another strategy a lot of people use.

I take your point, though. I can well imagine that once upon a time editors regarded this as an unambiguous grammatical error. I wonder when they stopped, though? How recent is recent? I suspect the shift didn't happen all at once . . .