Honestly, other than the D&D name on the covers, I, personally, don't consider Planescape, Spelljammer or Eberron to be D&D despite their inclusion of standard races and classes. Then again, go back to 1e and you had people claiming that Oriental Adventures was not D&D, because D&D requires a faux medieval setting. The same goes for both Al Qadim and Ravenloft: Realms of Terror in 2e. Given that I like Oriental Adventures, Al Qadim, and Ravenloft, who am I to tell fans of Planescape, Spelljammer, or Eberron that they are not playing D&D just because it does not meet personal aesthetics of a D&D setting.
I can totally appreciate where you're coming from. D&D can stretch, but, at a certain point, it breaks. I would never try to use the PHB to do a modern game. I'm not sure I'd even want to try to use the core mechanics for a modern game, even with new classes (d20 Modern just never "worked" for me).
Personally, I think that things like Greyhawk, Forgotten Realms, Dragonlance, etc. are unquestionably "D&D", without really even bending anything. The 2E core Ravenloft is pretty much there, as well. Oriental Adventures, Al Qadim, Maztica, and even Dark Sun still qualify as D&D, to me, but I also think they each would work best with a separate PHB that gave unique classes and races for those genres/settings (they're a bit of both). Really, the 1E Oriental Adventures was pretty much perfect in how it handled things; there was nearly no overlap (I might have just reprinted the Fighter instead of using the Bushi), but everything was 100% compatible and you could do Marco Polo or whatever.
I don't care for Planescape, but I think it also qualifies as being "base-line" D&D. By that, I mean both the underlying mechanics and the specific implementations of them found in the PHB, DMG, and MM (i.e. races, classes, spells, monsters, magic items, etc.) serve the setting well, supporting play without introducing cognitive dissonance. The setting just happens to be somewhat more fantastic than most of the others. It's really in the same boat as Ravenloft -- not vanilla, but still in-universe.
That's also where I'd put Eberron, speaking as an Eberron fan. In some ways, Eberron is the most D&D of settings because some of the core conceits of the setting are around what would things look like if the actual game rules were taken to logical conclusions, specifically, the existence of reliable, predictable magic. In other ways, the setting really bends things. The pulpy action that the setting wants to evoke could probably be done better in something like Savage Worlds (and Kristian Serrano did a pretty
well received conversion). Personally, I think it can be run on either side of that line. It works very, very well as "base-line" D&D, with a couple of extra bits (races, class, feats). Even though some folks insist on trying to play Eberron as some sort of pseudo-steampunk setting, that really wasn't the intent of the setting. As long as you don't do that, I'd say that Eberron is actually
more "base-line D&D" than Oriental Adventures or Dark Sun. As I said, probably about the same place on the scale that Ravenloft is.
I don't know that much about the other settings (Birthright, Spelljammer, Council of Wyrms). Anything I said would be conjecture. My gut is that some are pretty close to "base-line", but I really don't know.