I was just thinking of lost items of Felltower.
- Rangol Grot's mysterious stuff. The PCs kept trying to summon the lost spirit of Rangol Grot to get more loot out of him, but that's pretty much over. I can't see a plausible way for new PCs to try to pick up this thread. I'm sure one of my players will object, and point at the blog, and say people know about it. Which isn't ludicrous, but it also justifies other wizards in the world doing it, too, and then what? Will the new PCs win this race? Unlikely.
I think we can call Rangol Grot's spirit largely safe from summoning and interrogation.
- Dryst's key. Dryst's player shows no sign of wanting to play online, even almost 4 years into a drought of games. He's got the key to the otherwise-inaccesible door in the dungeon on level 2. It's not clear there is anything there worth getting at, but he's got the only way it and it's effectively out of play.
That also goes for the bone sword, Belt of Power, Staff of the Woodlands, Magebane, and a number of other items wielded by PCs whose players don't seem likely to make a comeback.
- the Black Library's books. Not actually lost - they're still in the dungeon. But the players aren't inclined to delve without a cleric. Not a single cleric anyone has run is willing to visit the library without attempting to destroy all the books. So, the books stay inaccessible. Not lost, but blocked by changed character approaches. We'll see if what's in there is ever really accessible.