I had promised myself to intervene but then it slipped my mind; it's been a while since I've read Doc... I hope he is well
In my opinion, regardless of the topic's placement in the Jazz section, there are the prerequisites to use the term even in the strictly classical world. I was thinking about what happened at the beginning of the last century thanks to Debussy (taken up by Stravinsky) and Olivier Messiaen.
To simplify, the Western method was that of development, of "succession." With the musicians I mentioned, it shifts to a type of composition by "juxtaposition," a form of panels or frames and therefore according to a cyclic model, which some call a "spiral."
In this context, in my opinion, one can speak of an initial rebellion against "inevitable progress," something that jazz musicians call precisely "Mainstream," meaning "main current," thus a "rebellion" against the mainstream and therefore a search for a second path, an alternative (and not a replacement, mind you!) to what... let's face it... was the German one.
To tell the whole truth, in my opinion, the first signs were already present in the Russian national school (see The Five), where however they relied instead on tradition.
In this context, open forms will become interesting; Claude Debussy's "La Mer" absolutely must be analyzed.
In this regard, there is an interesting essay that deals precisely with the birth of open forms.