|The disk ought to be physically OK as long as you've kept it away from magnetic fields, dirt or anything corrosive.|
Here's what MS says about the descriptor byte:
It says there the disk can be read by older Dos and windows 95. I guess those don't set the byte while later OS's do. But I'm pretty sure 98 will still read a disk without the byte set.
Or, as Outlander says, maybe it's from a Mac.
You're not really green until you're soylent green.