What is canon? The dictionary defines it as the works of a particular author or artist that are recognized as genuine. In the science-fiction and fantasy genres this is often used for different media and is not always the work of a single individual. In the medium of science-fiction novels we could say that the Eternal Champion has a collection of works that form a canon. In television we would say that Star Trek has an established canon made up of several series created over a number of decades (and this includes film as well). We would also say that Star Wars has a canon made up of the story told in its films and quite possibly a number of sub-canons if you include the stories officially recognized by its creator George Lucas as being part of the “Expanded Star Wars Universe” in the media of novels, comic books, animated television and video games. What all these examples have in common is that they are all made up of stories.
Stories come from what we might call a source, be it an author, artist or group of such individuals that are recognized as having creative control or licence that grants them the position of authenticity. Roleplaying Games or RPGs are different. Product developed for RPGs does not tell a story. The story is told by playing through the encounter (or "module", and I’ll return to this later) and building the story together with a game master who manages the campaign world, and the players who control the action of their player characters. RPGs are by design what Dan Cross, in “Gary Gygax’s Insidiae”, would call “story-latent”.
By design each “Campaign World” has an historical backstory just as each adventure might also. An adventure is made up of encounters and it is here that the story is told. Adventures do not come with pre-played descriptions that describe the story. The adventure is story-latent. Campaign worlds do not have a canon, they have an historical backstory and each unique campaign being run by an individual GM in that setting has a canon specific to it. That is why each adventure was called a "module", it is an optional add-on or plug-in that can be inserted to build the custom campaign story.
Thus even a shared group play scenario like Living Greyhawk has a canon, but it is not the canon of the setting i.e. Greyhawk, but of the Living Greyhawk Campaign (set fifteen years in the future from the CY 576 date where the Greyhawk backstory originally ended).
Updates to settings are fine but certainly do not need to be used in every campaign and in my opinion should be considered an optional backstory variant and not “canon”.