In comparison to German, English is very "situational" with its predicate nominative (see this question).
Suppose the rule is that the predicate nominative is only ever applied for sentences like the following:
It was I, who did that.
then it would be very logical if it were
"Who was that."
"That was I."
- Is this spoken by anyone?
- Is this considered correct by the ardent defenders of the predicate nominative?
- If the answers are "No." wouldn't it make sense to say that there is no such thing in the heart of the English language after all?