Not good.
million masks stockpiled by Canada for a rainy day...are expired
Most "expired" dates are to get you to buy more when it isn't necessary. Fine, send the masks to me. If any elastic is rotting and falling apart (which is highly, highly, highly doubtful), I'll sew new elastic on which Canada could do easily. Seriously, Canada, use them and don't put more of a strain on the world's shortage.
Yeah, I don't see anything in the 3M info. about storage date affecting the media. The concern is with the seal or a strap breaking. (Test those straps vigorously, I guess.)
Some foods with vegetable oil (etc.) in them can go rancid rather quickly after the expiration date, I discovered just a few days ago. Mom recently had some saltine crackers in what I thought would be near ideal storage conditions (unopened, low humidity, always between 68 deg. F and 75 deg. F): I opened 'em up a month past the expiration date and they already had that odd slight "paint thinner" smell, taste way off. I was really surprised. Checked her "Cheezit" crackers - same story.