Kinda, sorta though. If you go by simply core material, not the expanded supplements, the default cosmology is a bare skeleton with virtually no details. The abyss has 666 layers and demons live there. There are 9 layers of the hells and Asmodeus is the ruler. That's about it. What does the fifth layer of Hell look like? Until you get into supplements, there's no information. It looks like whatever you want it to look like. How big is each layer? Who lives on each layer, whatever, is pretty much left up to the DM.
The 1st edition
Manual of the Planes covers a bit more than that though, and that's the foundational basis of both 2nd edition's cosmology generally and Planescape specifically (though the latter does play with several of the assumptions within the book). Each layer of Hell gets a paragraph in the book and all of the Outer Planes get some pretty detailed sub-sections all of their own.
But I'll admit that whether MotP is "core" or not is a matter of opinion (it's certainly designed to be generic enough to be used in most campaigns though).
I mean, now kobolds are tiny dragon people tied to Tiamat whose god is locked away in a labyrinth. That's MILES from what kobolds have been in the past, but, these changes pass without any comment. Why is it perfectly acceptable to completely rewrite kobolds to the point where they aren't even recognisable - completely different societies, different origins and different associations - but changing an eladrin from super angel elf to playable character race is met with constant hue and cry and never ending criticism? Same with tieflings.
Why do Planar elements get preferential treatment?
If I may hazard a guess? It's a matter of perceived importance and what people are prone to getting attached to.
Kobolds have always been minor monsters. Indeed, until they started acquiring dragon-like characteristics they were explicitly little more than canon fodder for low-level characters to earn XP from. Until the dragon lore was stuck onto them there was little to distinguish them from any number of similarly low-level threats that disappeared as soon as players had accrued enough experience. They still more or less fill that role, but they're actually in some ways more distinctive than goblins now, who're now considerably more generic by comparison. So the change was for one thing (I believe) perceived mostly positively. More importantly though, it was to a critter very few people cared about at all.
Tieflings and eladrin through are very different. Tieflings were already a "cult classic" race before 4e came along. They were never core, no, but they were popular enough to jump ship from Planescape when the setting was dropped during 3rd edition and land in the Forgotten Realms, which kept most of their basic lore intact. And while I doubt very many people cared much about eladrin in pre-4e D&D they did fit into a cosmological order that had been around for a couple of editions and which people
did care about - which was the arrangement of the Outer Planes and the division of outsiders into celestials and fiends (among other creatures). Not to mention, of course, that 4e eladrin stepped very much on the toes of what people considered to be "elves," causing a lot of players to cry foul when it was suddenly declared that high elves and gray elves weren't really elves anymore but this new entirely different race that meant something else in a previous edition.
Let's not forget either that tieflings and eladrin/elves were
player races, which intrinsically attract a lot more attention than monster races like kobolds (yes, you could make a kobold PC, but I think it's fair to say they were even rarer than planetouched PCs in pre-4e). You can bet that if 4e had tried to radically change the lore and mechanics of dwarves, people who have been pretty upset as well (indeed, I was more than a little annoyed that 4e tried to rewrite half-orcs into some non-hybrid race that didn't require human/orc breeding).
In summary, people care different amounts about different stuff. It isn't so much that the planes are intrinsically more resistant to change - it's just that a lot of people care a lot more about the planes, tieflings, and elves than they do kobolds. Likewise, a lot of FR fans were really upset by 4e lore changes that had
nothing to do with the planes and everything to do with trappings of the Realms they considered sacrosanct.
From my perspective, it's not the players, but the supplements that I have a problem with. The fact that every single article MUST adhere to a single vision. Every module, every racial write-up, every splat book, every setting, must be shoehorned and forced into this single mould. No other part of the game gets treated like this.
I don't think that's really true. Warhammer and Warhammer 40,000 each have their own very specific lore, though it comes with the caveat that a lot of in-universe sources aren't very reliable. Pathfinder has a pretty defined setting in Golarion, complete with its own version of the Great Wheel. Both versions of World of Darkness have a unified meta-narrative (though each separate to the other). Eclipse Phase has a defined world setting. So does Shadowrun. And so on. I'd say a core setting where there's a single vision is actually the norm for most tabletop RPGs. The fact that D&D gives full official support to numerous different worlds is actually kind of unusual.
In other words, if I want to buy a module that takes place in the Abyss, I should be able to do so without that module having several pages of Planescape baggage attached to it.
I'm not sure how that would actually work actually. If it's a commercial module that's supposed to take place in the Abyss, it kind of has to commit itself to certain worldbuilding details, at which point it either conflicts with Planescape (or another setting) or it doesn't.
If I buy a Monster Manual, it's perfectly fine for there to be conflicting lore in it, IMO. What's wrong with the lore being unreliable? Why does every bit of lore automatically have to be taken as gospel canon? Why can't I get a lore writeup that says, "Here is one example of a kobold" and in the next Monster Manual get, "Here's another example of a Kobold that contradicts the first MM"?
I actually don't mind having unreliable lore but I think a lot of players (and GMs) expect there to be a defined "norm." And it's okay to defy that norm, but you kind of do want to have a baseline. I think a lot of people would find it a bit strange if the section on orcs had three different entries describing for orcs, depending on whether they were more similar to Tolkien's fallen elves, Warcraft's shamanistic warriors, or Warhammer's warmongering hordes. Different interpretations are usually setting-specific, such as Eberron's death-worshiping elves or Dark Sun's cannibalistic halflings.
The other thing is that every option demonstrated is still, intrinsically, something someone chose instead of someone else. Unless you're going to have infinite options (which is obviously absurd) you have to draw the line somewhere. At which point someone might well ask "why A, B, and C instead of X, Y, and Z?"
