Typically, if stored in tightly-sealed glass (in a cool/dark place), herbs keep a goodly amount of their potency for 2-3 years. Of course, powdered herbs 'lose more faster' than pieces.
Hmmm, Slippery Elm that's lost it's slippery. My best guess is that without the 'slippery' it's lost quite a bit of it's healing properties, but likely still contains some amount of it's nutritive value.
I'd throw it out (or add it to a mulch pile).
Good for you for 'paying attention' and noticing the difference!!!