Only as a bound variable pronoun, it's use as a singular personal pronoun is very very recent and not very common.
It is at least a reasonable claim that could be investigated further, as opposed to the claim that singular "they" is always an error, which any corpus will show is false.
That's possible but I've never seen any historical example of this nor I have seen anyone do it (outside of a specific subculture) so I don't think it's true: after all it's usually easy to deduce the gender of someone you haven't met from their name and if you get it wrong nobody (outside of that same specific subculture) will get really angry because of that.