No, only the Fossil server was down, because I did an upgrade and forgot to restart it.
(Why doesn't it auto-start on boot? Because I'm procrastinating moving the site's Fossil startup script to systemd. No, don't argue; is what it is.)
Fossil makes mirroring easy. Someone just has to decide to stand up such a mirror on a separate public substrate to allow such a fall-back. Given that, you'd switch an existing repo over to the clone with:
or similar, and be right back in business. And if you had no clone yet, you'd just clone it from the mirror site, of course.