If the newsletter uses a List-Unsubscribe header gmail will add a little unsubscribe button at the top of the email. It shows up in maybe 25% of the newsletters I receive.
CAN-SPAM should be extended to make it required for commercial mail to add this header.
I'm not so sure I agree. What happens when something better comes along? I don't trust Congress to know what's happening in technology today, let alone to keep legislation up-to-date with technical innovation.
Same for the various versions of Apple Mail. No idea whether there is a keyboard shortcut though.