I've read in a couple of places that freezing yeast puts it in a dormant state and it will last indefinitely in the freezer. That's why frozen doughs don't continue to rise in the freezer like they do in the fridge.
I have a large bag of yeast in the chest freezer - I put my 'ready supply' in one of those jars yeast comes in, and keep that in the fridge freezer.
it's never failed. likely 7-10 years in the freezer.
some people did some stuff - and rejuvenated 3000 year old yeast from an Egyptian tomb to make "real Egyptian beer" - as I recall, they pronounced it "quite terrible"
Yeast in an already baked item are dead, killed by the heat of the baking.i think that yeast in a jar or sour dough starter and yeast in an already baked item are different. i've had yeasted breads die on me whereas in a jar they stay fine.