"who was it" is correct and can be used in written English.
For example, "John heard a knock on his door, but when he opened it, no one was there. He wondered to himself, 'Who was it?'".
Exact(58)
If everyone makes it, the person who was "it" is "it" again.
LOGAN: Who was it?
Who was it from?
Who was it that knew him?
"Who was it?" "He didn't say.
"Who was it?" "Talking in my sleep.
"I said: 'O.K., who was it?
"Who was it?" "No.
Just who was it?
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Since I tried Ludwig back in 2017, I have been constantly using it in both editing and translation. Ever since, I suggest it to my translators at ProSciEditing.

Justyna Jupowicz-Kozak
CEO of Professional Science Editing for Scientists @ prosciediting.com