The wish for an unbind command is at least 12 years old -- I first noticed it in 2001. There's been recurrent discussion on what the right approach is, and despite having written the standard(?) unbind patch using the standard bind-to-noop approach, I think there's merit to the argument that a true removal from the keymap is better. But I don't think that anyone has ever implemented it. Meanwhile my patch has never been merged upstream, and I last updated it for mutt 1.5.6.
This updated version of the patch is for mutt 1.5.22, and it goes back to the fundamental issue. Instead of binding to noop, this unbind/unmacro actually removes the binding from the keymap(s). I think the behavior is at last complete, and pending review it should be considered for the feature branch.
