This sadly is a common theme for academic databases. Many databases were running on funding money. And after the funding gone, so is the database
One thing I don't like about COS preprint sites is how much they rely on JavaScript. I'm not sure the Wayback Machine can archive them correctly. It might take a specialized bot.
The Wayback Machine doesn't work right at all on one of my own COS preprints: https://web.archive.org/web/20200211103826/https://engrxiv.o...
Maybe I should contact ArchiveTeam...
The files can be downloaded directly by adding '/download' to the engrxiv URL for each preprint.