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