Singular 'they' isn't new; it's over 600 years old

Long before becoming a flashpoint in modern grammar debates, 'they' was already used to refer to one person of unspecified gender: the Middle English poem William of Palerne contains an early example around 1375, and Geoffrey Chaucer used it the same way in The Canterbury Tales in the 1380s. Prescriptive grammarians only began objecting to the usage in the 18th century.

— Standard linguistic and historical accounts, Singular they — Earliest attested use, William of Palerne, c. 1375