Borrowed from Old Norse 'þeir', replacing native Old English 'hīe' — one of the rarest events in linguistics: a basic pronoun replaced by a foreign word. It happened because OE plural and singular pronouns had become confusingly similar. Singular 'they' dates to the 1300s, not the 2000s.
The third-person plural pronoun, used to refer to two or more people or things previously mentioned or easily identified; also used as a singular pronoun for a person of unspecified gender.
From Old Norse 'þeir' (they), replacing the native Old English 'hīe/hī'. This is one of the most remarkable borrowings in the history of English — languages almost never replace their basic pronouns with foreign ones. It happened because the Old English third-person plural forms ('hīe', 'hiera', 'him') had become dangerously similar
Singular 'they' is not modern — it dates to the 1300s. Chaucer used it in The Canterbury Tales: 'And whoso fyndeth hym out of swich blame, They wol come up...' (c. 1395). Shakespeare used it too. The idea that 'they' must be plural is a prescriptivist invention from the 18th century, centuries after the singular usage was established