"Who art Henry?" was never grammatical English. "Art" was the second person singular present form of "to be" and it was already archaic by the 17th century. "Who is Henry?" would be fine.
The part of the lord's prayer that says "our father who art in heaven" is kinda like this - father is linked to a second person conjugation. You could remove some words and make it into "father art in heaven", which you claim is ungrammatical. I'm skeptical that it was.
Conceivably it’s grammatical if Henry is vocative and the pronoun is dropped colloquially, like “Who art [thou], O Henry?” but it’s a stretch.
Today, even ignoring the dated conjugation, "who art in heaven" or "who are in heaven", does not make sense. We would switch it into the third person.