Many make-up products will let you know when they have gone bad. The color changes, it smells bad, liquids separate into layers, black, white or green gunk is growing on the surface of liquids and more. If it smells bad it is bad. Any liquid with black strings or leaf like things in it should be discarded immediately. If a liquid has gone hard it s bad. If a cream has turned to liquid, that s also bad. If a powder has turned hard -that s not bad. It s called hard-panning and it s a common problem. There are videos on you tube showing how to soften hard-panned powders. Moat powders such as eye shadows, blushes and shimmers are made with minerals and moneral based pigments. These are inorganic and cant go bad. If, as you say -they have been stored away from light then many will still be good. Like you said, many expiry dates are cover-their legal-backside warnings with little substance to them. Let your nose and your eyes guide you -any changes in color, consistency or smell is telling you that the make-up is past its prime.
I have a couple eye shadows from the big 1999/2000 new years eve party in NYC. I used the very last of the silver ad blue shimmer shadow and it was as good now as it was back then. I also have a blush from 1995 -a bright pink -and it finally gave out. I put it on and it was a streaky mess -not bright pink.