My 5.25" ones from the mid-eighties are still readable. As far as I can tell the 8" ones are as well, though I haven't tested many of them. But their density is

My 5.25" ones from the mid-eighties are still readable. As far as I can tell the 8" ones are as well, though I haven't tested many of them. But their density is low, which helps a lot. And that's also why none of my 3.5" 1.44MB floppies are readable - the density is a bit too high for what's feasible for that physical medium. I don't know about later ones, but early CD-R easily failed after less than a year back when I tried to use them as backup media. I quickly abandoned the idea of using them as backup.