No. The settings materials represent "A" starting point, but never present themselves as "THE" starting point. When we look at Eberron, for example, Keith Baker even now will repeat the golden mantra of Eberron: "If it exists in D&D, it has a place in Eberron." That mantra suggests that there is not a definitive canon - no "THE" starting point - but a potential starting point for your adventure in the setting. Again, this is about creating a starting point but not defining itself by a hard canon. Keith Baker has also said in reference to Eberron that he intentionally left a lot of questions, secrets, and plots regarding the setting open for the GM to create their own. There is no THE canon when it comes to the Draconic Prophecy, and Baker refuses to answer as he himself does not know.
IMO this is akin to claiming space is a vast dark emptiness... well yeah if you ignore the planets, gasses, stars, asteroids, etc. that populate it, I guess it is. In other words if you choose to ignore the things that are sst in the Eberron campaign setting then yes it would appear that it doesn't define "The" starting point for an Eberron game of course that's ignoring all the things that are filled in and specific to Eberron (which create "the" starting point for a canon game) such as...
1. It takes place in the aftermath of the Last War.
2. There are 5 human dominated nations
3. 1 nation was destroyed and has become the Mournland'
4. Dragonmarks exist
5. There are dynasties built around said Dragonmarks
6. Dragonshards exist
7. Drow venerate scorpions
8. Magic is common place
9. There are 7 continents
10. The world has 12 moons
11.Kalashtar and Warforged exist in Eberron
12. Shar the City of Towers exists in Eberron
13. Do I really need to keep typing what is defined in Eberron (and thus the starting point for a canon Eberron game)... it would take all day.
EDIT: And you do realize that the fact that The Draconic Prophecy exists in Eberron (even if it's details haven't been fleshed out) is in fact establishing Eberron canon...
I choose to tell you the truth of how I would respond to your hypothetical question. I do find your question somewhat absurd from my own experience. In my years of gaming and talking D&D settings with other D&D fans, I have never been asked about any notions of canon in the setting. I imagine that if I did talk about any sense of "canon" for a setting, it would actually turn-off a lot of those people from those settings entirely. Maybe that's because most gamers I play with and discuss tabletop gaming with have a sufficient understanding that D&D settings are gaming toolkits and not dogma.
You find the question of canon from a D&D fan... absurd?? Now I find that anecdote strange as there are whole forums on the internet populated by D&D fans who discuss exactly that. But fine, it's not a question you would answer... though I fail to see how wanting to know about the canonical state of say Eberron or DL or Dark Sun as it stands currently necessarily leads to D&D settings as "dogma" rather than toolkit. If you are using Dogma in the sense of... that which is known to be true... I fail to see how a campaign setting can't serve both purposes? It can both set what is known to be true before play and serve as a toolkit and springboard during actual play...or do you disagree?
Yeah, I'm afraid that you're not understanding me correctly. Your question seems to mistakenly presume that there is only ONE canon and one gaming community for a D&D IP, which I said does not exist. There will certainly be A community that adheres to the canon imposed by the owners of the IP (and that community will likely attempt to impose that canon on others), but it is never the only canon that exists. This is because there are canons. Plural. These are canons dependent on the communities. Again, plural. Communities determine canon, and people, institutions, and groups attempt to impose their canon on others. But there are multiple understandings of canon based in the particular communities of gamers. Some may hate, for example, the Spellplague. Some groups refuse to acknowledge the Spellplague as canon. Others who may hate it will hate it, but regard it as canon because they tend to adhere to something of a blind belief that everything created from X authority source (i.e. "official") is canon. And it sounds like, from my own understanding of your situation, that your adherence to a singular notion of "one true official canon" from WotC (pbuh) is a boon to your online gaming needs. This canon - as tacitly agreed upon by your gaming group - allows you to game easily with each other online because you share a common set of setting assumptions. This approach requires less thinking. There is one canon; the canon simply is. But that is not how canon exists, because, to stress this point again, there are differing notions of setting canon in differing gaming communities.
Wait...what? So now anyone with a "community" (whatever that means) can in fact create canon for anything they want?
So correct me if I'm wrong but what you seem to be saying is all those who claimed D&D 4e wasn't D&D... were right (Note... even as someone who wasn't particularly fond of 4e I still consider it D&D). Anyone with a community can proclaim Hyperborea, Middle Earth or the Young Kingdoms as official D&D settings and they are canonically correct?? I don't think it's a failure on my part to understand you, it's that I think you are using the word canon incorrectly and I'm choosing not to agree with you. A group refusing to acknowledge the Spellplague as canon in no way makes it any less canon.
EDIT: Also... "requires less thinking"?? Please expound because I'm not sure I agree with such a broad and general assertion... does it require less thinking than say a game where the setting is creating through play and decided on by the rolls of dice? Does it require less thinking then mixing and matching? In what areas exactly is there less thinking involved?
We could also use a less religious set of literature for another example: what is canon in the Harry Potter books? Some people take everything from JK Rowling as canon: books, tweets, tv appearances, Pottermore, etc. Others only take what is written in the books as canon operating on the principle of "the author is dead." This is not to mention the remnants of the Shipping Wars whose various "ships" have their own sense for canon. Many people will adhere to the "JK Rowling Canon," as it is the largest community and it is regarded (at least by those within that community) as the "official" canon. But belief in a singular canon does not make it so despite what JK Rowling (and others) would otherwise have you believe.
Uhm... JK Rowling (as long as she owns the rights) decides what is and what isn't canon. It's pretty simple really.