Well, by the official definition of "canon," shouldn't it only be defined after the work in question is a dead issue and you can actually sort the apocrypha out from the canon in a final and definitive manner?
No, because it's not something that needs to be decided. There is no "official definition of canon." It has never been an officially designated label. It's a metaphor people use to refer to a comprehensive and complete body of creative works. It's a description, a nickname. It doesn't define what something is, it's just a handy way to refer to it.
The canon is just the totality of the work itself. It means all the creations that constitute essential parts of a body of work, whether they're continuous or not (for instance, the Shakespeare canon is all of the literary works confirmed to be authored by Shakespeare, even though few of them share a continuity). If something is outside that canon, that means it isn't an essential piece of the work, but is optional or derivative, a secondary creation by a different party.
Granted, things like TV series have multiple creators, but history shows that it's virtually impossible to make tie-ins canonical unless the creators of the actual canon are directly involved in their creation. So canon is almost always a very simple and obvious thing to define: It's the work of the original makers, as opposed to imitations by others.
I'd expect "The Star Trek Canon" in the traditional definition would include stuff like "Yesteryear" and "Vulcan's Glory" and omit "Threshold" and "The Alternative Factor," which is a can of worms none of us wants to deal with. Honestly, the thing relating to Trek that most suits the original concept of "a canon" is probably something like the novelverse flowchart.
Vulcan's Glory is a novel that contradicts quite a few later and earlier Trek episodes. Despite being by D.C. Fontana, it never quite fit with the way TOS portrayed Vulcans or their presence in Starfleet. So there's no reason to consider it part of the canon. It wasn't written by Fontana in her capacity as a Trek TV series's story editor, but as a freelance author contracted by Pocket Books.
I'll concede that it is possible for authors to declare that a certain one of their works is no longer part of their canon, i.e. no longer considered an essential piece within the whole. After all, my upcoming novel
Arachne's Crime replaces my first published story "Aggravated Vehicular Genocide" within the canon of my original universe. So I'll grant that "Threshold" has been decanonized by its own authors. But such things are rare exceptions, not routine practices. In general, it's enough to understand that a canon is a specific comprehensive body of works. It's not something that has to be litigated in every single case; most of the time it's going to be very straightforward.
And canon and continuity are usually distinct concepts, because many canons have discontinuities within them that they don't bother to resolve. Many canons include stories that contradict each other and pretend they actually fit, like the Marvel Comics canon with its sliding timescale. It's rare that creators actually bother to come out and say "That story no longer happened." Usually they just pretend it happened in broad strokes but differed in details, or just tiptoe around the question of whether it happened. A canon is a
whole body of works, and a flawed piece can still contribute to the whole even if it's mostly glossed over. For instance, "The Alternative Factor" has mostly been ignored, but it's the original source of the term "dilithium," which is essential to the Trek universe. So it's not accurate to say that it no longer counts within the canon. It made one major contribution to the canon, just not to the continuity.
