There's different versions of jQuery. If you require a version that supports older browsers, you might not be able to use the bleeding-edge version. The current libraries will continue to work as they always have. There's no reason for newer versions to be limited by outdated browsers. Developers are free to choose the version they want to use.
New versions do not need to be limited by old browsers. Why not build a mutable API? Your browser doesn't support canvas? Oh, too bad, but the Canvas module will just be undefined so you, the developer, can choose what to do with your user.