Hussar
Legend
And the DMG when it talks about the planes, and in pretty much every supplement in 3.x that focused on the planes, and in Dragon/Dungeon material as well. It was omnipreset. If you wanted to write planar D&D material that was one of the major sources you did your homework on since it was the most in-depth source covering the topic.
It's one the issues with writing for a shared world, you need to be aware of the material and its continuity if you want to write for it. The material has been a part of D&D long enough that it's iconic, and even as part of a core, implied setting and common bag of tropes and themes, it approaches shared world status as a baseline to be knowledgeable about, even if specific campaign settings can deviate from it. If something is changed, it's usually for a very good reason and with popular support, and if it's not, there's going to be a giant amount of blowback from the fanbase.
This, right here, is what bugs me.
Why does Planescape get this? Why do we have to have Planescape as the default planar setting? I mean, it's a bit circular isn't it? You have to write to Planescape to get published, because Planescape is the only planar setting, but, if you try to go in a different direction than Planescape, to, maybe, flesh out a different source than Planescape, you won't get published. So, Planescape is the most in-depth source because any other source automatically gets strangled out of the conversation.
Which is precisely what I've been complaining about all the way along. No other options get even entered into consideration because Planescape exists in this privileged position where any deviation is automatically edited out.
No other setting gets this level of control over core material. I don't have to write to Dragonlance lore when I write about Dragons, nor do I have to care about Council of Wyrms or any other source of Draconic Lore. In fact, I'm pretty free to contradict anything I want. Like I said to Imaro, dragons have gotten a massive rewrite in every edition with no apparent problems and considerable approval.
But deviate in the slightest from Planescape lore and it's impossible. We MUST keep everything Planescape compatible and we're not even allowed to consider changes.