The database dump e621 publishes still has metadata for deleted files, so you could automate importing of files with an FA url via the FA archive (I was experimenting with try to convert pools to comics, but I'm hesitant to dump all of them on the site at once).
That'd still leave all the stuff from when 2 year old commercial stuff was still allowed. A lot of that stuff is going to stay missing unless someone who was ripping the site in the early 2010s still has everything and is willing to share. You could find a lot of stuff on u18chan digging manually, but that site is in a constant state of disrepair that I'm surprised the owner even bothers to pay for hosting.
PurpCat