We have to remember that the first generation was learning as they fought in the dungeon so anything they think is non-essential will be forgotten unless someone figure something out.
Yeah, a feather that opens a return portal to the surface with a piss easy activation method that could easily have been figured out by chance is totally non-essential.
Again, this isn't something that required research and development to refine and use. It's a magical feather that appears in a flash of light, SOMONE would have messed around with it to figure out what it did (which someone clearly did at some point) and the manga expects us to accept that not a single First Generation explorer accidentally bent and dropped one of these feathers, saw a portal open up that teleported them to the surface, and spread that potentially lifesaving information around?
That's the "logical leap" I'm talking about. If the Feather required 6 Goblin Magic Stones, spit from a hydra, blood of 16 virgins, and the moonless night on the 6th day of the month, unless that day is a Thursday, then sure, I'd accept Gramps not knowing about it. But crumbling it in your hand and tossing it behind you would activate it, and no one did that? Come on...
Edit: The last I'll say on the topic is this: If the feather were like a Phoenix Down in Final Fantasy where the sole function is "revival after death" I could absolutely accept that there were decades between discovery of the item and the accident that revealed it's use (i.e., someone dying while having a feather on them). Which would then lead into the "can it work twice? Or was it a once per person thing" which no one would volunteer to test if the magical feather would revive them a second time. So a gap in knowledge between "it's a useless feather" and "this is a life-saving tool" makes sense, especially given the First generations "we're fighting for our lives" mentality. But the return Feather could have been activated by a single person going "Another feather? Trash." and crushing it in their hand and tossing it on the ground. That's not obscure, and it's easy to test, and it's a "Get Out Of Danger" card. It's a minor nitpick, it doesn't ruin the manga, but it is flawed logic if you think about it more than "harr harr, gramps has been throwing ¥30,000 in the trash"