Why do I need a migration plan? I have jquery.js saved on my disk; that's never going to vanish, and it's not going to stop working if upstream decides to stop supporting it. And if I discover a bug then I can just fix it myself, just like any code saved on my disk.
jQuery rarely has security issues anyway; and the issues that do exist are usually low-impact. It certainly won't have any more than any code I'll write myself.