Apparently "it's too good" so the US government slapped export restrictions on it, saying it wasn't allowed to be used by any foreign national, within or without the US.

https://www.anthropic.com/news/fable-mythos-access

why shut down?

The official story is that it's so awesome the US government didn't want anybody else to have it.

My current theory is that their business practices will quickly be recognized as fraud in any country not owned by the AI companies, so they are cutting that risk out.

It completed my entire todo list, nailed everything, wrote fantastic tests, simplified code, fixed a stack of vulnerabilities, cleaned up a bunch of tech debt and that was all in one evening

I'm a product manager in software development, and I'm vibecoding some stuff as a hobby (like i made chartsgoup.com/ because I'm trading stocks and I wanted a convenient tool for myself).
Usually working with Claude is like with a real developer - you ask to do something, it does the work, it's almost alright, so you write a list of stuff to correct, check the result, correct again, etc, until it's done.
With Fable it was suddenly like this: "Do this" - "Ok, done, and I also found problems in my initial code but they are also fixed." So the majority of the usual back-and-forth was removed, the quality is really good. I am not sure if any random person is able to make a good app or web service now (experience is still needed to formulate your ideas, plan stuff, make decisions, etc), but it's much closer to that. And it's already answered that US govt thought that Fable is too good for non-americans, Anthropic couldn't really make it work only for US citizens like that, so they shut it down until the situation clears up