Since people have asked for it, here it is... (This is the original text, from August of 2003)
Of late, there has been much verbiage expended on the topic of canon, and what should and shouldn't be. I think that some of the folks expending the verbiage are forgetting that which is of critical importance in any discussion of Canon...
Canon is for Authors
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An Author is not an ordinary person; an Author is someone who is writing material that will be Given the Imprimatur. Such people cannot with impunity go against What Has Been Established, for the Imprimatur says to one and all, "This is part of What Has Been Established.". Thus, all Authors need to be familiar with the material under the Imprimatur, that is, the Canon.
Most of the people on the list are not Authors - although we are blessed with the presence of some who _are_ Authors, and even with those who may Give the Imprimatur.
For players, all rôle-playing games are is an evolution of "Let's Pretend" - "Let's pretend that I'm a cowboy and you're an indian and I'm gonna stop you from scalping the women of the town and bang! bang! I shot you and you're dead -- " "Am not! You missed!" "Did not! Cheater!" "Did too! Cheater yourself!"...
All that the funny dice do is provide an impartial arbiter of whether or not you missed. All that the pages and pages of rules do is provide the information you need to understand what the dice are telling you. And all that the pages and pages of source material do is provide Imagination, collected and distilled, to establish the context in which to interpret the dice to determine whether you really did miss...
And no, none of that is trivial, else we'd never have had the "You missed!" "Did not!" arguments when we Pretended before we started RPGing. But it's still that simple.
What then does de-canonization of material mean? It simply means that the material in question need not be considered when an Author writes to Receive the Imprimatur. If what an Author writes has nothing to do with the de-canonized material (as in a Solomani Rim sourcebook not needing to consider the astrography of the Glimmerdrift Reaches), then the de-canonization of the material is of no moment. If the Author writes about what is covered by the de-canonized material, however, it becomes important - for Canon must be followed, and the Story shoehorned into the material that Has Gone Before - but if the material is de-canonized, the Author need not fiddle with the Story to fit the material, but may change the material to fit the Story, without fear of Losing the Imprimatur thereby. If the Author chooses to write his Story such that the de-canonized material is still accurate, then none will criticise him thusfor, nor if he choose otherwise, and write the material anew and differently.
To a player (which includes the Referee, the arbiter of the reality of the Story), the canon status of any material is Irrelevant. For the player, the first rule of the Rôle-Playing Game is "An ye like it not, CHANGE IT!". Canon or not, the Referee is Telling a Story, Interactively, and the Story is all. The conformance to Canon is a convenience, not a necessity.
Then what purpose the discussion of Canon? "...Imagination, collected and distilled,..." The discussion of which we partake on the list is part of the process of collection and distillation, providing a common basis for sharing our Imagination. To do this without reference to what has Received the Imprimatur would make our shared experience that much more difficult to achieve; thus, the Canon provides that common base upon which to build - and build we do, else would the formulation "IMTU" be devoid of meaning. Even so, Canon must always defer to IMTU in the final analysis, for when we play, we make Traveller _our_ Game, and none shall gainsay us that power.