"Singular they, along with its inflected or derivative forms, them, their, theirs, and themselves (also themself and theirself), is a gender-neutral third-person pronoun. ...
Singular they has been criticised since the mid-18th century by prescriptive commentators who consider it an error. Its continued use in modern standard English has become more common and formally accepted with the move toward gender-neutral language. Some early-21st-century style guides described it as colloquial and less appropriate in formal writing. However, by 2020, most style guides accepted the singular they as a personal pronoun.
In the early 21st century, use of singular they with known individuals emerged for non-binary people... They in this context was named Word of the Year for 2015 by the American Dialect Society, and for 2019 by Merriam-Webster. In 2020, the American Dialect Society also selected it as Word of the Decade for the 2010s."
https://en.wikipedia.org/wiki/Singular_they
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