Chill, canon is malleable. Stuff can be added or struck from it. TAS used to be entirely non-canon, now portions of it are.
TAS was never really non-canon. Roddenberry and his continuity enforcer Richard Arnold issued a memo claiming it was no longer canon, but at that point they had no actual authority over the shows themselves, since Roddenberry had been shunted into a mostly ceremonial consultant role due to his deteriorating mental state. The actual shows freely made references to TAS (e.g. "Unification" mentioning "Yesteryear" backstory and DS9 mentioning the
Klothos) during the period when the "ban" on TAS was supposedly in effect. The only things actually affected by the "ban," aside from public opinion, were the tie-in books and comics during the time when Arnold was in charge of their approval.
Assignment: Earth, as a backdoor pilot, would be a good candidate to be removed from canon if it already wasn't so isolated an episode.
I'd say the opposite -- since it's so isolated, it's less likely to conflict with anything and thus there's less reason to worry about its canon status. Things that get "removed" from canon tend to be things that are directly contradicted by other canon -- "The Alternative Factor"'s bizarre interpretation of antimatter,
The Final Frontier's portrayal of speedy travel to the galactic center, "Threshold"'s version of transwarp. "Assignment: Earth" is such a self-contained piece that it's unlikely to come into conflict with anything else, so there's no reason to single it out.
Again -- continuity is not the standard here. In TOS, in pre-'80s television in general, lack of continuity was desirable. Episodes weren't
supposed to depend on each other, since there was no guarantee that a given viewer would be able to see all of them, or to see them in any consistent order. So the priority was to make every episode work independently. In a very real sense, there was no "canon" and no desire to create one. There was just a series of individual, unconnected stories about the same characters and premise. A show like
Mission: Impossible would sometimes have the team expose their faces on national or global TV in one episode, yet still be able to function as anonymous undercover agents in the next. Or they would bring down the head of the nation's top crime syndicate in one episode and yet organized crime would be as strong as ever in the next, and the next, and the next. Plus there were far more nameless Eastern European countries and generic People's Republics than could possibly fit in Eastern Europe. No continuity, no consequences -- each episode was a universe unto itself. (Some shows continued this practice into more recent times -- for instance, there's no way that all the murders in a single season of
Law & Order could've possibly been investigated by the same two detectives and tried by the same two DAs, let alone within the course of a single year.)
Canon is whatever people want it to be.
As I said, that's the exact opposite of what the word literally means.
By definition, no. "Canon" is a shorthand nickname for the work of a series's original creators or owners, a word that's used
specifically to differentiate that original work from outside creators' derivative works such as licensed tie-ins or fanfiction. If there were no outside knockoffs, there'd be no need for the word "canon" at all; the word only exists to discuss the difference between the two.
As a rule, the only tie-in books or comics that manage to work as canon are those that are written, plotted, or directly supervised by the original creators themselves -- for instance, the
Buffy comics made without Joss Whedon's involvement during the run of the series were not canonical, but the comics directly supervised by Whedon after the show ended were canonical. Because canon is not about the medium, it's about who creates the work. No two artists will interpret the same subject in the same way, so the only way you can really get a reasonably consistent creation (for no long-running series can be perfectly consistent) is if the same person is in charge of all of it.
