As Apple notes on its support page, it's been a long time since Quicktime was necessary for iTunes -- that changed with version 10.5, which was released way, way back in October of 2011. And while Quicktime used to serve up a web video, it's similarly been deprecated and replaced by technologies like HTML 5. So for most users, there's basically no reason to have the app installed anymore. Given its status as a potential security threat and the lack of any updates coming from Apple, we'd go so far as to say you should get it off your computer as soon as possible.
We've reached out to Apple to confirm that it has ended support for Quicktime on Windows and will update this post with any other details.