For example, we can go to the Wayback Machine at archive.org to not only see what a website looked like in the past, but prove it to someone (because we implicitly trust The Internet Archive). But the Wayback Machine has deleted sites when a site later changes its robots.txt to exclude it, meaning that old site REALLY disappears from the web forever.
The difficulty for a trusted archive solution is in proving that the archived pages weren't altered, and that the timestamp of the capture was not altered.
It seems like blockchain would be a big help, and would prevent back-dating future snapshots, but there seem to be a lot of missing pieces still.
Thoughts?