Why is it "what" here instead of "why"? Are both "why" and "what" acceptable here?

"What" here is used to make the point that it does not matter, while "why" is asking for reasons why it does. That's my understanding of it.

"Why does my death matter" is a question that presupposes that at least someone in the room thinks that the author's death matters. "What does my death matter" is a rhetorical question or a statement about how the death doesn't matter. I don't think you can make any sense out of it grammatically, you just learn the phrase. The basic form is "what does it matter?" where you can replace "it" with pretty much anything. If you want to ask an actual question you just cut the "what". Does it matter?

Both are correct. “What” seems a bit more poetic in this case.

What is asking how much?  Why is asking in what ways, or for what reasons.

Bearing in mind that this will be a translation from her speaking in German, either is acceptable. Might change the nuance a bit, but you’d be understood

“What does <<it>> matter” is a common phrase that basically means “why is <<it>> important”. There’s a song called “The Sound” by a band called The 1975 that uses this phrase, if you would like to hear it in a different context.

What is short here for "to what extent". Why being used here would have a different meaning. The most frustrating thing about this sub is people answering out of their ass CONSTANTLY. NO, they're not the same.

its great

"What does it matter?" Is kinda like saying "Does it even matter?" Whereas "Why does it matter" is more asking for specific reasons why it matters. Usually this construction is used to indicate that the speaker thinks something doesn't matter, often with a condition. Like "What does (x) matter when (y) is true?"

slightly different take, i think the equivalent why sentence is 'why would my death matter.' both of those constructions are rhetorical, inviting the listener to think about the consequences. 'why does my death matter' sounds more direct to me. i'd think the speaker expects a response, a reason or an admission that there are no reasons. the difference between 'what does' and 'why would' is mostly stylistic; to me these are generally interchangeable.

Either one is acceptable. What does my death matter: this is asking does it even matter - leaning toward really stating that it does not. Why does my death matter: leans a little more toward asking for a list of reasons why her death matters. Sometimes I hear "What does it even matter" and it's rhetorical, not actually asking, more throwing hands up and saying it doesn't matter.

In this example what seems to mean "to what extent" https://en.wiktionary.org/wiki/what#Adverb > what (not comparable) (interrogative) In what way; to what extent. > What does it matter? What do you care?

As a native English speaker this just sounds right, but I will give it my best shot to explain it isn't referring to the person and how they matter but the how the death matters If I'm wrong please tell me