Mm. I'm a bit sceptical of the historical expertise of someone who thinks that "Who art Henry" is 19th century language. (It's not actually grammatically correct English from any century whatever: "art" is the second person singular, so this is like saying "who are Henry?")
That text was from the example prompt, not from the models response
The problem is the subjunctive mood of the word "art".
"Art thou" should be translated into modern English as "are you to be", and so works better with things (what are you going to be), or people who are alive, and have a future (who are you going to be?).
Those are probably the contexts you are thinking of.
As a reader of a lot of 17th, 18th, and 19th century Christian books, this was my thought exactly.