I’m sorry, but it’s not “very well attested” that window-curtain was a common phrase. According to Google ngrams (which you also cited, so presumably trust), those two words used together has never been particularly common, any more than you’d expect from straightforward collocation.
The word curtain has come to - except in the theatre - nearly always mean the kind you use on a window. The meaning has become narrower. Agreed. And it interesting the way words change meaning over time, from general to narrow, or from one specific type of narrow meaning to a different but related one.
But it’s not a case of a word dropping a modifier that was used as part of the word, like motor-car dropped the word motor. “Window-curtain” with or without the hyphen just was never very common as a phrase.
Funnily enough I was actually looking recently for some door curtains, and it was annoying because everywhere comes up with regular curtains (they have slightly different features), so I do understand what you mean about the meaning of the word having narrowed in general use. What I dispute is the claim that “window curtains” was ever common. And that’s pretty much indisputable, or should be. And it means it’s not an example of a compound word that dropped its modifier.
So I’m not sure if what you’re thinking of is a form of semantic narrowing, which would apply to the way the word curtains has changed, or something more specific about dropping a modifier.