If you enter a domain, you will find which instances block this domain.
If you enter a reason, you will find all instances that blocked for this reason and who they blocked.
If you reverse search, you will find all instances the domain you entered blocks.
I believe most or all are blocked because of their open signup policy, which makes it easy for bots and trolls to brigade.
Thank you fedifriend! That adds much needed context and makes perfect sense.