As far as I'm concerned the use of "they" whenever it's uncertain whether "he" or "she" should apply has been part of English as long as I can remember.
ISTM that pronouns are rather more complex than singular/plural. In a nutshell singular is one aspect of being more personal and the plural an aspect of the more impersonal.
For second person the "plural" form has become the norm. Thee/thou/thine were the "singluar" form but the actual usage was governed by rules similar to tu vs vous in French. Use of "they" is entirely consistent with that.