Anecdotally, I used some yeast yesterday that I found in the back of the refridgerator with an '06 expiration. I proofed it and it foamed up, so I went ahead and used it. Seems to be fine. I agree that it's a good idea to proof it before you add it to all you ingredients though.
I bake a lot and use instant yeast which comes in a large package. I keep it in the freezer.
I recently ran out and opened a new package. Although I almost always proof the yeast and it always rose, there is a significant difference in my bread results from the old box and the new box. I will give away about half of this yeast to some other cook.
You might try just adding more yeast. Personally, I feel the effort involved in making bread is worth the cost of having fresh yeast.
Frankly, I was shocked at how much better the new box rose and tasted.
Just proof it before you try to use it.
Drop a package (or a teaspoon) of yeast into a quarter cup of warm water (between 105 - 115 degrees works) along with a teaspoon of sugar. Stir and let it rest for about five minutes.he temperature you'd use for a baby's bottle. If it feels uncomfortably hot, it will probably If it foams, it's active. If it doesn't, toss it out.
Usually, 90 days isn't unreasonably long for holding yeast over it "use by" date. Especially if you've kept it refrigerated. Just use it up quickly as it will not hold indefinately.
Proofing is definitely the way to go.
Then store your dry yeast in the freezer. I buy huge bags that last me years. The first thing I do is open the package and pour it in a jar I can open and close and that will keep it dry in the freezer. So I have no idea what the expiration date may have been and am not the least concerned about it.
Do NOT hedge your bet by adding additional yeast. Plan a longer rise. So long as you have a few live yeasts, they'll colonize in the right conditions. You don't want the flavor of the yeasts themselves -- you want the flavor of the alcohols and sugars that live yeasts produce as byproducts. So put your dough in an airtight container like a Cambro tub and let it rise in the fridge overnight.
The next day take it out and let it come up to room temperature. Take it out and knead it to give it fresh air. Let it rise again. Then use it when it's doubled -- this is about volume NOT time so let it have the time it needs. You'll get delicious bread.