Mercurius
Legend
OK, so there's nothing official about "core rules settings." I hear your perspective about differing from the core rules would represent a change from Greyhawk in terms of the settings relationship to the D&D rules, but you could also say that adding newer elements that weren't present in the original presentation is also a "change from what Greyhawk was in the past." It depends upon how you want to look at it. Your perspective emphasizes the crunch, mine the fluff.Core rules means "doesn't have setting-specific rules". Greyhawk has always used core rules. Gygax never excluded any races, for example.
Now, it would be possible to re-invent Greyhawk as a non-core rules setting. There is clearly a demand for a gritty low magic setting for 5e, and they could make Greyhawk that. But that would be a change from what Greyhawk was in the past, not a continuation. Anyway, GoS indicates WotC still consider Greyhawk core rules.
Dark Sun is not a core rules setting, never has been.
I personally like the idea of preserving "legacy settings" in the form in which they were originally presented, at least for the most part. That's why I suggested a Greyhawk product that presents the classic setting but with appendices about incorporating newer ideas as options (e.g. dragonborn). By updating every legacy setting to include every idea that has come after it was conceived ends up with a kind of homogenization and runs the risk of lessening what makes that setting unique.