verb agreement - Shouldn’t “art” be “is” in “Our Father who art in heaven”?

The Lord’s Prayer begins in English:

Our Father who art in heaven,
hallowed be Thy name,
Thy kingdom come,
Thy will be done
on earth as it is in heaven.

Shouldn’t it be who is there, not who art? You would have said thou art and he is at the time this was written. See the Wikipedia article on Thou for example.

Answer

"Our Father who art in heaven, hallowed be Thy (your) Name," is the rest of that sentence.

By saying "Our Father... Thy..." you are addressing God personally, making that the second person singular (you are). (First person singular: I am. Third person singular: he/she/it is.) "Our Father" is not speaking about God; it is speaking to God. (It is like saying, Hey, Dad, you, up there. Blessed be your name.)

If it were about Him, it would certainly be He is. From the same book which states Our Father, who art in heaven:

For the Lord is our judge, the Lord is our lawgiver, the Lord is our king; he will save us. - Isaiah 33:22

Quoting your source:

When thou is the grammatical subject of a finite verb in the indicative mood, the verb form ends on t... (e.g., "thou goest"; "thou dost"), but in some cases just -t (e.g., "thou art"; "thou shalt")

and

Originally, thou was simply the singular counterpart to the plural pronoun ye... thou was later used to express intimacy...

and

The familiar form is used when speaking to God... (an "informal" singular form of the second person in modern speech.)