In case anyone stumbles across this, the problem was that the entity that manages .icu domains received an abuse report and suspended my domain.
This is strange because I only use the domain for personal experimentation and none of that is just for static websites. DNS records for mail are properly configured like all of my other domains.
I asked Namecheap what the reason was. They said none was provided and it may take some time to get a reply back upon requesting the info. I asked them to put in a request for the reason.
My guess is that I was just in a massive list of domains that someone started reporting. or maybe someone was trying to get my domain taken out so they could take it for themself.
@Sam_uk, nice! Thanks. I was on an IP somewhere in the US (VPN). So maybe I got a different result today being on a different IP. At least I know what is going on at kbin.world now. It all makes sense :-)
@Holyginz, I would think you can do like on kbin, since it seems like the standard. Just guessing. Go to the user page on your instance then follow them.
Good question. Not ignorant. The one thing we use all the time but never think of as federated is email. Multiple users from different providers can reply all and everyone gets a copy. That is one way to help think about how the fediverse works. People using kbin and Lemmy have replied in this thread. We all see it. we are just using different providers/software to read and reply in the fediverse.