I have just read in Emma by Jane Austen a phrase which surprised me:
I can think of but one thing – Who is in love with her? Who makes you their confidant?
(Jane Austen: Emma, Chapter VIII, Obreey PocketBook)
Here the lattest who is clearly in singular (followed by makes), the gender of the person under question is unknown, so I understand it as a gender neutral their:
Who makes you his/her confidant? Who has confided you the information about her lover?
I wouldn’t expect such an old usage of their functioning as a singular gender neutral word. I’ve always considered this "grammar trick" as a much more modern invention.
Is it really a usage of the gender neutral their? Is it so old that Jane Austen could have used it? Or was it just some recent editor who didn’t consider the good old Austen "correct enough" and "fixed" it?
As the answer to the more general question of whether singular they is proper English notes, the construction enjoys a long history of usage in English.
In fact, the Wikipedia article on singular they has a section on the history of its use, and specifically calls out Austen as one of the respected, pre-modern authors who used they with a singular antecedent:
Older usage by respected authors
They was already being used with a singular antecedent in the Middle English of the 14th century. It is found in the writings of many respected authors, including Chaucer, Shakespeare, Jane Austen, Thackeray, and Shaw:
“I would have every body marry if they can do it properly.”— Austen, Mansfield Park (1814), quoted in Merriam-Webster’s Concise Dictionary of English Usage.
“Had the Doctor been contented to take my dining tables as any body in their senses would have done…”— Austen, Mansfield Park (1814)