The current explanation for the 'history' tag is:
The study of how music has developed and changed over time.
So that's the centre of gravity for what this site regards as a history question.
Some examples of what I think that includes are:
- Technical developments in instrument building
- Well-documented stylistic developments, and their (documented) motivations and influences
- The function of music in society, and the influence of that function on musical form
- Developments in notation
- Scientific discoveries relevant to music theory
- Origins of musical terms and conventions
- Biographical facts about prominent composers where those facts are relevant to their music.
Looking more specifically at your questions:
Why did Xenakis compose music that 'hurt' listeners?
This has been closed as off-topic, and I can understand why that might be confusing, as we already have other questions relating to composers' motivation and critical analysis of their music (as shown in your list - though it's been stated a number of times that not all existing open questions should be taken as 'good examples'.). I think the reason why the question might be difficult is that it seems to be based around one opinion given by one author; It may therefore be hard to give a good answer without venturing deep into opinion. However, if it really was well-documented that Xenakis did want to cause pain, and this was a significant point in development of music, then this question could well be on-topic, because people could give more factually-based answers.
Is Xenakis's dissonant music intended to express his personal sufferings and torments?
Again this question is based around only one 'citable' critical opinion (the others you mention are from redditors), and again I think it comes down to depth of documentary evidence - if there were interviews and other primary sources that could allow an answer to the question, then it could well be on-topic here.
Of course there's a logical difficulty here in that it might be hard to know when asking a question what the depth of documentary evidence is. One possible solution to this is simply to leave a question open and unanswered, but Stack Exchange does encourage that sites ensure their open questions are answerable.
How does ignoring the math behind Xenakis's cacophonously dissonant music, affect your reaction to it?
This is a little bit like your question on hearing the perfect fourth as dissonant, in that it seems to be primarily about using listening techniques in order to perceive music differently. This is an interesting area of question, but I think you might be ahead of the curve of human knowledge here - I'm not aware of a large body of accepted techniques for allowing one to hear or feel music differently, and hence it is probably true that at the current point in time 'answers to this question will tend to be almost entirely based on opinions', even if it's a reasonable question in itself.