Gmail does use List-Unsubscribe: when you mark as spam a mail which has a valid List-Unsubscribe header, it will ask you if you want to unsubscribe instead, and

Gmail does use List-Unsubscribe: when you mark as spam a mail which has a valid List-Unsubscribe header, it will ask you if you want to unsubscribe instead, and will handle the unsubscribe request automatically.