BTW, this is very useful:

https://open-slum.org/

It seems to be an instance of Uptime Kuma, which is a pretty great OSS for uptime monitoring and Dashboarding.

https://github.com/louislam/uptime-kuma

This site is down or inaccessible to me. What is in it and why is it useful?

That site has a list of shadow libraries, whether they are still operating, and where to find them.