Would evidence about masks wearing be one example? At the start of the pandemic it felt very surprising to me that we didn’t have a definitive scientific answe

Would evidence about masks wearing be one example? At the start of the pandemic it felt very surprising to me that we didn’t have a definitive scientific answer to whether we should wear masks or not. You’d think something so basic would be known. I’m not that familiar with the evidence so maybe it did exist but the scientific community was unsure about supply.