Examples, examples, examples. We all can come up with any number of perfectly on-topic questions beginning with either word at a moment's notice. A single word alone does not determine whether a question is on-topic or not, be it the first word in the title or any other word.
In fact, after just a quick check, 4 of the top 30 questions of my own begin with "is", and 6 of my top 30 answers are to questions that do likewise. That's 13% and 20%, respectively.
Nine out of these ten questions are open, and have 222 upvotes between them to boot. The top answers have 347 upvotes between them.
Similarly, 8 of the top 50 questions ever asked, or 16%, begin with "is", with hundreds upon hundreds upvotes between them:
And that's just a quick look at is. I am confident we can compile similar stats for questions beginning with can. Or do, which you haven't considered. (In fact, the two top answers on this site are both to questions beginning with do: Do most languages need more space than English and did English ever have a formal version of “you”.)
There is no denying some of these are MultiCollider bikeshedding flukes. But others are just as undeniably among the best this site has ever produced.
Most importantly, exactly none of these have received a simple yes/no answer, or would have been done justice receiving one and leaving it at that.
All that said, certain things — like not being able to spell "English" correctly on a site all about English, or titling your question "is this grammer??!!1" —, are certainly better indicators for poor or off-topic questions than others. But remember that even the poorest of questions can be saved by a great answer, possibly accompanied by an edit. And that's no hippie talk, that stuff actually happens. Indeed, the most recently introduced Explainer, Refiner, and Illuminator badges are right up that alley.
Conversely, there's no shame in admitting the greatest of questions can be shot down by a bunch of poor half-assed one-liners.
Anyway, I'd like to end this by saying that it'd be a nice, but fully surmountable, challenge, to edit every on-topic question on every SE site to begin with, end with, or include any word of your choosing. One word does not a swallow make. Don't get obsessed about symptoms, go after the malady.