I really like this idea. I used to work in advertising/marketing and we were launching a lot of purpose built, short-lived landing pages, e.g. to collect entrie

I really like this idea. I used to work in advertising/marketing and we were launching a lot of purpose built, short-lived landing pages, e.g. to collect entries in a contest, then announce the winners. These websites were live for a period specified in the contest terms, then usually archived. I could see myself routinely implementing this header in such cases. Also, from the resource consumer point of vie this could help guide efforts like archive.org to make snapshots before it's too late.