I'm using the spoiler tag because elsewhere I've seen this warning about the site:
run by people from kiwifarms (a known bad actor on the fediverse).
I'm unfamiliar with "kiwifarms" but the site currently does has some offensive language at the bottom that's associated with 4chan, so be warned.
That said, it appears to be the only site currently available to see which instances in the fediverse have blocked/defederated your instance. (The reverse of <instance domain>/instances, which only shows which instances your instance has blocked/defederated.)
They have a habit of doxing trans people for simply existing on the Internet. Also they've been responsible for several people being swatted. They were so awful that Cloudflare finally stopped having anything to do with them. Things have to be really bad for Cloudflare to nope out, it's a short list also containing the Daily Stormer and 8chan.