(If this isn't the correct place to ask this question, please point me in a more correct direction. I just didn't think there was an Ask Linguists sub)
So, first let me ask, I've heard that "they" in the past has been acceptable as a gender neutral, single person pronoun because English otherwise lacks one, and more recently it has been criticized and frowned upon. Is this a true statement? I know there's some debate on whether it should currently be accepted, but historically, was it actually the case that "they" has been used in such a manner?
If it is the case that "they" has previously been gender neutral, is there a general time period when the opinion of that use of it started to change? I know there isn't going to be an exact date, but was there an era when it started to become criticized and fell eventually fell out of favor? I know it's kind of a case of the red to blue spectrum and asking which is the first purple or blue letter can't really be answered, but is there a cultural or social movement or a period of time where we can say "this is the purplish bit" or "it's around here that it's not really red"? I hope that analogy makes some amount of sense.
You should ask at /r/linguistics if you don't find an answer here. That's the best place you're going to find for actual experts working in the field providing answers.
Also, "they" is still very much acceptable and in wide use. The notion that it is not is more of a pet peeve that you're encountering. It's not one that's codified or datable because it's been a long running pet peeve. Remember that there is no one standard for English. Much of what you would have learned in school is not actually "right" or "wrong", it's just the way your teacher felt you should be writing/speaking/etc. Never has there been a single standard of "correct"; such ideas are only arbitrarily chosen and enforced and are themselves constantly changing.