Should we flag the question about "Which episodes of Anime X are canon/filler?" as off-topic.
because there is a website AnimeFIllerList
Or shouldn't we ?
Browsing that site, it is very far from complete. Not that this site will ever be a complete database of anime filler (we aren't supposed to be), but it doesn't seem like that one will be either. Even for many of the shows listed, the list of what is filler/canon is incomplete.
Additionally, I think we can provide better context with our more free form responses than that site. For example, if you look at their Bleach list, and compare to what I compiled, you will notice that my list has additional information not on that site, including the fact that many of the filler anime episodes are actually adapted from manga side-stories or spin-offs, which may be interesting information for many readers.
And also, I think we often underestimate the difficulty of determining whether something is actually canon or filler. "Canon" sometimes is used to mean "existed in the source work", but that is not the literal meaning. For example, the first season of the anime Saki has a 3 episode arc covering the individual tournament, which is actually canon but originated in the anime; however the source manga later references it several times. So in some cases a simple binary canon/filler list is not sufficient explanation, but that is all the linked site provides.
Finally, I don't think the existence of an external site which covers some similar topics should have much impact on our policies here. Other sites will come and go, and may or may not be easy to find for casual searchers. We shouldn't expand our scope beyond Q&A to cover random other anime things, but we also shouldn't restrict it based on what other sources happen to be doing. If we did follow that approach, the vast majority of questions here could also go on /r/anime or MAL forums or some other places and hence there would be no need for this site. But by having it here we present it in an easily searchable, editable, and ranked format, which is often more useful than the alternatives in practice.
So I think the existence of an external site which can answer some of these questions is not very relevant for our decision-making here. If we are going to ban this type of question (which I'm not in favor of; they seem to be some of the most helpful and well-received posts when they go well), it should be based on how well they are working on this site, not what any other site is doing.