This question does not appear subjective and argumentative to me.
Its merely subjective, but I don't see any point about which to argue.
I think the requirement of the question, a source for which contemporary composers are innovative and which are not, is by it's nature both subjective and argumentative, as any such list would not only be subjective, but create fierce flame-wars. Hence, it is "impossible to objectively answer this question; questions of this type are too open ended and usually lead to confrontation and argument".
If the question had asked for a source of new contemporary composers, and each person themselves had to then decide if the composer was innovative or not after listening to them, then the question would have been fine. But by requiring that you judge which composers are innovative or not, the question becomes impossible and should therefore be closed.
The question asks not for which composers are or are not innovating, but rather which composers are considered by the contemporary art composition community to be innovating. Answers should not be an opinion, rather facts about a specific group's opinion. Thus, it is objective. It meets none of the guidelines for a great subjective question not because it is a poor subjective question, but because it is not a subjective question at all.
It's easy to see that the definition is subjective, especially in regards to music composition. There will be no right answer to the the question (Real Questions Have Answers). There will only be debate. What one person sees as an innovative application of Baroque tonalities in a theme others may see as a limitation of musical expression.
The question was closed correctly.
The opinions solicited are not opinions based on supportable experience, but flat out, plain, vanilla, opinions. Thus, it is prime flame-bait.
Now, as a would-be community in beta, it seems to me that we have the option here of experimenting a bit. If someone posts a question like this, and it gets three or for sober answers and no emo extravaganza comment soap-operas, then we could keep it and others like it. If it turns into a tomato throwing contest of fanbois arguing about the relative innovativeness of their favorites, then it needs to be terminated ex poste haste, and any more like it likewise.
In other words, it's not 'what has helped you learn to play the sackbut', but rather, to overstate the case, 'are republicans or democrats better?'