Why is it insufficient? If the site doesn't want to leak that information, it doesn't participate in cache sharing. Since sharing is opt-in, sites won't unknow

Why is it insufficient? If the site doesn't want to leak that information, it doesn't participate in cache sharing. Since sharing is opt-in, sites won't unknowingly leak this information. Edit: whoops. I see what you mean. I missed an edit while modifying an earlier draft and left the opt-in only on one side, the sharer.