The need for it is situational. Many countries will demand to see it if you are coming from a country with yellow fever and will not admit you without it--note that it's not the yellow fever countries that care! However, if you're not going into yellow fever territory it's irrelevant. I have no idea of where mine might be now, if it even still exists (I expect it doesn't--my parents would have put it away after our last trip and I think those old documents are lost.) It's been 35 years since I set foot in yellow fever territory, I have had no reason to care about it. If for some reason I was going back into the yellow fever zone I would get a new vaccination anyway.