This isn't true though, they has been used for centuries as an indefinite singular pronoun, like "oh someone left their umbrella, they probably want that back l

This isn't true though, they has been used for centuries as an indefinite singular pronoun, like "oh someone left their umbrella, they probably want that back let's see if there's a tag". It is the case that using it as a definite singular is new, and we'll see if that lasts.