I did some quick digging on another post and will put it here too.
With covid we are dealing with three slightly different viruses with 3 vastly different rates of spread which has resulted in guidance changing (not science changing its mind or flip flopping).
R0 is roughly the rate of spread so with OG covid 1 person would give it to 2.79 people, etc.
OG Covid R0 was 2.79
Delta Covid R0 was 5.8
Omicron R0 is not known but compared to measles which is greater than 7 and some have compared the spread to measles which is 12–18.
So, yes, guidance changes.
Cloth masks could be effective in effectively slowing OG covid enough because OG covid was not as contagious and cloth masks, while not perfect, combined with social distancing could make a significant impact in getting the R0 closer to less than one or one — which means that the spread would be slow the closer to one it is and it would stop spreading eventually if it drops to less than one.
Then with Delta cloth masks helped bring the rate of spread down and combined with vaccines and natural immunity we could deal with it.
But Omicron is crazy contagious AND vaccine effectiveness as far as getting it and spreading it has dropped. (It reduces both but not as much as Delta AND it still prevents serious illness.) With an R0 this high, if cloth masks brought the R0 down 70% that would still be AT LEAST 2.1 which is very close to the rate for the original unmasked covid (and it could still be higher if the R0 for Omicron is as high as some people think.)
So, cloth masks do still help and they helped before.
The issue is that this is so contagious and it takes so little to spread that cloth masks, while cutting down on the spread, still leave us in an untenable spot because the virus will still rage out of control. (Particularly since vaccine effectiveness also is not as good.)
That is why they changed the mask suggestions. N95’s or KN95’s may increase the level of protection significantly AND, unlike before, we have them available more easily.
Scientists did not just suddenly change their minds on masks.
The virus changed and they realized the impact of the change — and modified their recommendations accordingly.