Is asking a "yes or no" question a violation?

No

Not as far as I'm concerned......

These questions are knowledge-seeking but really don't require more than a simple yes or no:

Do I need a passport to cross into Canada?

Can I use shortening as a substitute for butter in a recipe?

Is the metric system still used in Europe?

Are pythons venomous?

Is there an extra charge for a window seat on a plane?

No, but unless the question has chatty overtones. That would be a violation if it's reported for chat.

It is not automatically a violation, but history of the site shows that such questions are usually violations (mostly chat), so they should be avoided. Always ask for some facts in your question.

Yes

No shouldn't be

it is arguable - the answer citing the knowledge rule is generally correct

but - polling often asks yes/no questions

should guns be banned? - is a legitimate poll in my opinion - even though it is likely to get yes/no answers

should guns be banned - why? - add the knowledge seeking component ( most questions can avoid gray areas by using the how, what, when, where, why formula)

i think in general many of those questions should not be reported

some of the ones asking about penis size and other adult chat should be

The guidelines say the question is to be one requiring a good knowledge of the subject by the answerer.

A question asking for a yes or no answer means it was not "requiring knowledge".

A " yes or no , maybe, Idk " answer is not a knowledge requiring answer.

You have to read between the lines in the guidelines.

*** It would appear that one can get away with these kinds of question/answers in the Polls and Survey category.