Is I the correct pronoun in the sentence it was not I who left the door open?

Yes, the pronoun "I" in the sentence is correct.

The pronoun "I" is functioning as the predicate nominative following the linking verb "was".

A linking verb acts as an equals sign, the object is a form of the subject or restates the subject.

A pronoun used as a predicate nominative is in the nominative (subjective) case.