Sort of:
Lothar the Master of Bones. Very unused chap, though he was present in Planescape: torment. Very high level cleric of the abstract concept of death who lairs within Sigil and largely doesn't have much to do with anyone. He stays to himself and doesn't get into the politics of the City of Doors. Besides, it's probably better for his longevity if he stays that way.
I've never presented him, or anyone else, as a 'wise high level NPC to help the PCs when they need it'. But by virtue of him being wholly neutral and a veritable treasure trove of knowledge if you're willing to give him something in return, my PCs have gone to him from time to time for obscure bits of information. However it has never been at my instigation or even suggestion.
He was there for the three years of my first campaign, and now in my second campaign (150 years later in character) he's still there.
And then there was Tollysalmon, the former factol of the Bleak Cabal. However she was totally insane and scary, so they only asked her anything on one or two occasions. And she too is still around.