According to the Oxford English Dictionary, singular "they" has a long history going back to 1375.

https://public.oed.com/blog/a-brief-history-of-singular-they...

With a little googling you can verify that Shakespeare, Jane Austen, and Chaucer all used it.

I have to disagree, while it can sound a bit "off" I think it's probably the most natural word to use where gender is unknown or irrevelant. All the other constructs I'm aware of that have been used for this purpose are more of a speedbump than "they", for example replacing "he" with "he/she" or "s/he" rather than "they" is a much more tortured way of achieving the same meaning in my opinion.

If thou wants to be consistent, drop the singular "you" as well.