Focus Sash is not nearly as good as Sturdy is. With Sturdy, you can actually equip an item like Life Orb and Z Moves in order to boost your damage output. Focus Sash + Sturdy will actually be healthy for the metagame as every Pokemon will have a way to check problematic sweepers like Blaziken.So if we ban multiscale and sturdy (which ruins stall making HO dominate even more) then we would also have to ban focus sash as it literally does the same thing as sturdy. Magic Guard is clearly the broken thing that enables these three things to become very good.
My opinion may change in the future but I am strongly against banning it for now. I don’t believe Magic Guard is an inherent problem, but the consistency of it and Sturdy/Multiscale makes it an insanely spammable ability. With those abilities banned, it won’t be as mandatory and as dominant as it is right now. Multiscale being banned would also go towards aiding the stall vs stall mirror. It’s still a very toxic matchup, but now stall teams can run a powerful wallbreaker to somewhat threaten these types of teams. Semistall would be a much more dangerous playstyle.