Given how central and important canon is to certain franchises like Star Wars, it makes sense to ask and answer canon-related questions on the main site. However, the canon itself is not one and the same with the actual in-universe discussion that is the domain of the main site and therefore should probably be separate. One could argue it either way.
They belong on both...
Questions about what is/isn't canon have a place on both the main site and on meta, but for subtly different reasons:
They belong on the main site when asking about what "really happened" in a creative work. As you say, this is the kind of thing that's of great interest to fans of a creative universe. It's true that "Canon" is a meta-concept: it defines the universe of discourse, but isn't itself a part of that universe. But we've always maintained that out-of-universe questions are on-topic on the main site, so I see no problem here; it's still a question about how a particular SFFnal universe works, and so belongs on the main site
They belong on meta when asking about what makes a good source for answers. For better or for worse, as a community we've largely agreed that answers without "canon" sources are, at best, less desirable; that inevitably leads to the question of "Well, okay, but what is a canon source?"
This, fundamentally, is a question of how to interact with the site, and belongs on meta.
...Personally, I'd rather have them on Main...
The difference between those two points above is largely one of intention, which is difficult (if not impossible) to determine in most cases. If I were to offer advice to a new asker, though, I would recommend they post their question on the main site1, for two reasons:
- They're more likely to get better answers
- More people will benefit from the knowledge shared
Which are really just nuances on the same reason: more people visit the main site than meta (by an enormous margin).
...But it's going to be case-by-case
There are situations where such a questions belongs on the main site, and other situations where they belong on meta. For that reason alone, I'm not comfortable setting down a commandment on this topic; it depends on the question.
1 Provided it was a good question for the main site, not too broad and on-topic and so forth
Both, in essence.
- If a question about canon is about a specific universe it needs to be asked on the main site and have the appropriate tags added.
- If the question is how we treat canon or the mechanics of using canon in answers, it should be on meta.