So for what it's worth - I've been using JAV-IT
over the last couple of days to download my library from R18 before the closure. I forgot I bought this thing forever ago, when I was first looking for a way to strip the DRM from R18 videos. Never got around to doing it, so if there's one minor upside to R18's closure, it's that it gave me the kick in the ass to get on converting my library.
It's a tedious process going through each video one by one to get the id. Would be simpler if the ID was just the JAV code, but R18 usually sticks some extra numbers here and there so you have to go to each individual stream to make sure. But this thing is like $5, which is nothing to save your library. So if you've got some basic command line skills and any R18 vids you want to keep, I'd recommend buying this little utility and start downloading and backing up.