The Far Realm gets a few mentions in Lords of Madness and the Epic Level Handbook and once in a while gets trotted out in Dragon.
Spelljammer gets a controversial reinventing in Polyhedron, a tiny mention in Lords of Madness, and, IIRC, one article in Dragon since the aforementioned Polyhedron article.
Mystara/the Known World gets... um... even as a Spelljammer fan I have to admit it gets better treatment than this. Every great once in a while Dragon throws it a bone, and that's pretty much it. Has a single creature originating in this setting made a Monster Manual from WotC?
Birthright doesn't even get as much as Mystara.
Dark Sun gets a (widely criticized, although I liked it) reboot in Dragon and Dungeon and a very occasional bone thrown it it in Monster Manuals. Despite being one of the main 2e Epic settings (the main Epic book for 2e was in the Dark Sun line!) it got no mention in the ELH, skipped in favor of more planar stuff. To be fair, it does have a very slick fan site, though.
Planescape? It gets Deities and Demigods, Manual of the Planes, Planar Handbook, Fiendish Codex I, Fiendish Codex II, a strong presence in the Fiend Folio and Epic Level Handbook (heck, a strong presence in MM1!), mentions in the DMG, third-party reimaginings from its original creators, and constant support in Dragon and Dungeon...
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Aside from the Realms, the only 2e setting that gets even remotely the kind of support Planescape does is Dragonlance, and that only because its creators keep it going via second party products.
Personally, I can think of at least 20-30 setting- or style-specific books I'd rather see than a Fiendish Codex III.