Mykos out of date , Can it stil be used?

Should last 20 years if stored properly, "use by" dates are mainly for stock control. If it is only out of date recently, use it with complete confidence, there is no magic ingredient that will neutralize a product overnight.

Mushrooms survive almost anything. See The Last of Us 😄

if its in dry, i would not worry. maybe not so active, but if storing was right, its should be still potent.

If stored properly it should still be fine. Maybe won’t be as effective as it was when it was new but it doesn’t mean it’s completely gone bad. But if it worries you, get some fresh stuff for piece of mind.