I think that's kind of dubious logic, but I do think a Planes/Planescape book is a decent place for these (almost no groups would object to it, as almost every D&D setting uses the planes and Planescape is basically canon, though in the sadly "Monte Cook took a giant crap all over it" format rather than the beautiful thing Zeb Cook - no relation - gave us). Whereas putting them in some book full of Greyhawk stuff would be enraging.
I'd be unsurprised to see WotC split the difference, actually and have "X's guide to the Planes", with a sort of Xanathar-style personality guiding us through the Planes. Just so long as it's not some GOOGLY-MOOGLY has-been Forgotten Realms "celeb" like Volo. He can go back into retirement. Bring us a REAL NPC from Planescape. Someone who says "berk" non-ironically.
(Note: portions of this post may not be intended seriously - adding this before someone decides to defend Volo's "honour" or something equally mad!)