Well if you're inquisitive enough, Google can be your friend, and today I learned that indeed 3M face masks have a shelf life of 5 years. This is shorter than I would have thought. Anyone interested can read more here: https://multimedia.3m.com/mws/media/...f-life-faq.pdf
Of course there are suspicious types like me that wonder if the shortest possible shelf life might be in the best interests of the manufacturer who would like to sell as many as possible. But I suppose that's a whole other conversation.
I would bet 3M and their competitors in the space will be selling a lot of face masks and PPE in the future regardless of the recommended shelf life.
Edwin