Nothing particularly rare, perse, but I did keep Shadow’s Edge, Deathbringer’s Will and the Quel’Delar on my DK, all gotten when Wrath was current. Just couldn’t bring myself to sell all that, so they’re chilling in my bank.
I’ve got gladiator mounts, old PvP tabards, CM sets, TCG mounts and tabards and a bunch of other removed items, but the rarest thing I still have is probably the elite thrown weapon from s9 in my rogues bank.
It’s a grey item now since they removed the ranged slot from rogues, but it used to be a real item and the 2200 version even had its own appearance that differed from the standard throwing axe from that season. I doubt anyone really saved theirs since not only did transmog not exist yet, but you couldn’t transmog them anyway. I would imagine basically everyone that had one vendored it when they got the next seasons item. Dunno why I stuck mine in my bank, but its there.
I’d love to see if someone has the elite appearances from the Cata elite wands, I dunno if they had unique appearances too, but wands weren’t transmoggable for many years after Cata so it’d be interesting to see if anyone kept theirs and could use it now (assuming there were wands to transmog to)