Knock sensor on 04 Forester

I've heard people from this group saying there's a knock sensor on 04 Forester that went bad sometimes. I asked the same question to my mechanic and told him that I put in regular unlead gasoline as recommended on the manual. He said cars with knock senser should use premium gas or else the function of the sensor would be useless. Cars without knock sensers can use any type of gas. Is that correct I wondered? He means there's no point putting in a knock sensor if regular unlead is used. The knock sensor will become faulty if you keep using regular unlead instead of premium gas.