[asveikau]

In some languages you can put a second person conjugation next to a noun that might otherwise use third person verbs, and it serves as implying that you are that noun. I'm not sure if older forms of English had that construct. I think many Indo-European languages do.

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.

[canjobear]

“who art in heaven” is a grammatical relative clause because the subject of the verb is the relative pronoun “who” which is second person in that context. You can still get this kind of thing in modern English, for example “I, who am a farmer, will be happy” is grammatical because the relative pronoun “who” is first person there. That doesn’t mean it would be grammatical to say “*A farmer am happy” and it wouldn’t have worked with art either.

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.

[asveikau]

I think the further back you go in Indo-European grammar, the more common the thing you are describing becomes. For me it's less of a question of if English did this, and more like how far back you need to go.

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.