I think any casual passer-by in MSE interested in intellectual freedom and openness would be appalled that questions get closed as "not about mathematics, within the scope defined by the help center" with no attempt to explain why the question was out of scope. In nearly all cases, I suspect, it is not that the question is not about mathematics, but that it doesn't pose a mathematical question in a way that those who close the question feel is appropriate to MSE.
These closures are value judgments and, in my view, should always be accompanied by a more specific explanation as a courtesy to the person whose question has been closed (even if that explanation is just "you have posted a zillion stupid questions about this boring topic and we are fed up with it, you and your arrogant ways" or "your question has received more than X close votes and so has been closed as a result of a democratic procedure that you can appeal against as follows ...").
I don't want to raise a question about a particular question, but I think an example is helpful. https://math.stackexchange.com/questions/1919791/my-attempt-at-proving-an-unsolved-problem just got closed on the authority of just two users, with no adequate attempt to explain why. The question is certainly a mathematical one and closing it merits some explanation, even if it's just "we can't be bothered to check claims by 19-year olds that they've solve the Collatz conjecture".
Can someone please explain to me how these unexplained closures helps the community? The impression given is that MSE is a dictatorship that legislates on what is and what is not mathematics with no public disclosure of its processes or of how they are applied in individual cases.