Singular "they" is attested back to the 14th century. (Note that this would actually be
Middle English!) It met a lot of resistance in the 19th century by various prescriptive grammar books.
https://en.wikipedia.org/wiki/Singular_they
https://en.wikipedia.org/wiki/Singular_they#Trend_to_prescri...
https://en.wikipedia.org/wiki/Linguistic_prescription