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- Panel 1: A person with the text "Singular 'they'" written on them smiling with open arms.
- Panel 2: "Singular 'They'" beaten up by others who said, "Singular they is ungrammatical. It's too confusing," "How can anyone use plural pronouns for singular," and "Every pronoun should only have one purpose."
- Panel 3: "You" hiding from the mob who was beating "Singular 'They'"
- Panel 4: "German 'Sie'" hiding with even more fear next to "You"
What's funny about those "grammar purist" people is singular "they" has been accepted common use in English for centuries, even older than singular "you". For some reason society got it in our collective heads in the fairly recent past that it was improper grammar, though, and that's what teachers often teach. I'm still not over my 5th grade teacher marking me down a point on an essay because I used singular "they". You're still wrong, Mrs. B.
Like many things, the damage was done by the British. Specifically one Bishop Robert Lowth. In 1762, he wrote a book of prescriptivist grammar rules starting with the premise that Latin is a perfect language, and any construction in English that doesn't match Latin is a flaw. This is where those nonsense rules like "never end a sentence with a preposition" and "never split infinitives" come from, as well as the claim that the singular they (in common use at the time) should be phased out in favor of the generic he, because that's what Latin does. The damage this one book did to the English language still has not been fully repaired.
Bring back thy/thine
You're confusing two different usages. Singular they to refer to an unknown or undetermined person has centuries-old usage, yes. Using it to refer to a known single person is an invention of the last few years.