Without a scientific trial (even if a double blind trial won't be possible, a randomised trial should be) in a non-medical setting, masks shouldn't have been mandated, particularly in settings that had been open with the vast majority of people not wearing masks until it was mandated, for the length of the pandemic.
A large scale randomised trial would be difficult, I'd have thought, since the principle to be tested is that masks reduce transmission from an individual.
So we can't allocate everyone in Manchester to be a mask wearer or non-wearer and see if they are more or likely to be infected.
We'd have to compare different places (so how do we compare?) or look at different times (but again other things are likely to have changed).
Difficult.
Does that justify just telling everyone to get on and wear one? Not sure.