Kbin was a project from a solo developer who also ran the main instance but had his real life (health issues, IIRC) demand all his time in the past few months. He had to abandon the project and the instance. Since it was open source it got forked and is continuing under the new Mbin project.
This is one of the crazy parts of the Fediverse that I don't understand yet.
I am aware of Kbin and was under the impression that it was very popular. I haven't been there but how would I know when an instance disappears or where would I be able to find out where it's been picked up later on?
Imagine an email server dies forever. All of the email that it sent is still out there. If you are on another server, any emaills to/from people on that server are stil in your local email storage.
Likewise, a post that was made to kbin.social is still out there, if someone outside of that server was subscribed to that kbin.social community ("magazine") when the post was made.
To find out the "new" server... well in an ideal world the old server would at least have a notice to that effect. But this case involved a medical emergency, so that didn't happen.
Click here for an additional detail which may be amusing
Now look at: https://lemm.ee/c/13thFloor@kbin.social This is the copy of the same community, but made for the lemm.ee instance. You'll note that it is missing a post that was made on July 6th. That's because the July 6th post was made by a user on lemmy.world. Because kbin.social was down by then, the post didn't "federate" from lemmy.world through kbin.social to lemm.ee.
See https://lemmy.world/c/synthwave@waveform.social for another example. waveform.social was an instance that died, but people were still posting to the lemmy.world copy, but only lemmy.world people were able to see those posts because they were not federating. So in comments we just decided to make any new posts to another community on a "living" instance, and we put a notice to that effect.
Interesting, the Lemmy World 13th floor link takes me to K-Bin because I'm using the Voyager app. Normally, it's nice that the app handles those links this way, but it seems, in the case of instances which have died, where commenters want to link and show people new posts that have been happening, the Voyager feature is actually a shortcoming.