There were two studies that took place during COVID. One was a trial of 5,000 Danish people who were split into either being instructed to wear a mask or not. The other study took place in Bangladesh and was a study of 340,000 people. Cochrane Met Analysis is known as the gold standard for combining studies about medical interventions. But I think the problem is that in most cases these are looking at things like drugs where studies are easier than actually doing them.