And yet, the rule exists. If you use “to be” in a conditional sentence, you have to use “were”, even when the subject of the clause is a third-person singular subject.

In counterfactuals, not conditionals.

If he were there, I would have seen him. (Counterfactual subjunctive.)

If he was there, I didn’t see him. (Conditional indicative.)

If he were here, I could see him

If he is here, he’s invisible.