If it's a GET, ATS should retry the request on the next available server. POST is a bit trickier, ATS shouldn't retry if any bytes have been sent but that won't be the case if the server is really dead and doesn't accept a connection. There's a finish number of retries and if all of those fail, the error will then go back to the user.

Will that "save" the first request to the dead origin? (or would the
error make its way back to the user?)