I thought of this idea for a thread when reflecting about "New Coke." I'm too young to have tried it when it was briefly in restaurants. When I was a kid, you could still see it in convenience stores as "Coke II," but I didn't know about what it was at that point. I largely missed out on "crystal pepsi" as well.
I think my answer would be Hershey's S'mores bars. Those things were good, and I didn't realize until recently that they were discontinued. For a while I just thought they were really hard to find.
They should have like a company or website that solely produces and sells discontinued snacks and drinks. That would be cool.