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.
The 3E DMG doesn't, as best I recall. The 3.5 DMG may have been different in this respect.
Other sources did, I'm sure, but they're not core, are they?
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.
I don't write for RPGs other than the material I prepare for my own game.
From the point of view of a player/GM like me, D&D is
not a "shared world". I am not trying to ensure that my game maintains continuity with anyone else's game. I don't need continuity across my
own campaigns - in my current (4e) campaign I follow the core cosmology pretty closely, whereas in my previous campaign (Oriental Adventures run using RM) the chief "evil" outsider was a fusion of Asmodeus, Demogorgon and the Ordainer (a demon from RM), and the planar structure was a homemade version of one RM scheme.
what is the point of having lore if it changes with every edition?
So we can run different games? To try and to speak more powerfully to potential new players? In the case of 4e, the lore was part of what brought me back to GMing D&D.
I agree with your first assertion
<snip>
If new lore is created it invalidates games... so that's really a non-point, it's going to happen regardless if any specific lore is chosen.
You agree with my first assertion. And in the other bit I've quoted you seem to reiterate my main point, namely that any lore, even Planescape law, will invalidate something somewhere.
And that's all I'm saying. Or to flip it around, you seem to be agreeing with me that Planescape lore is not in any special, neutral, universal and hence unchangeable category. The decision to keep it or drop it is simply a decision about whose games to validate and whose to invalidate.
What choice WotC makes is of course up to them. If they think Planescape is where the big money is, they'd be silly not to go for it. That doesn't mean that they're not invalidating other games, though. Opting for Planescape isn't opting for universality. Even if we put 4e to one side it wouldn't be, because Planescape isn't neutral or generic with respct to original AD&D either.
You can also have maximal lore, which posits a lore that makes no strong decisions about what is, and simply offers up a buffet of selections for individual groups to decide upon.
Fine, but in that case the mere fact that some option would contradict Planescape wouldn't be a reason not to add it to the buffet, would it?