Tried to update a tor relay on a machine running Ubuntu Artful, but the repos at https://deb.torproject.org/torproject.org artful/main are gone missing. Any ideas which repos I should use instead? Thanks!
you should instead upgrade the machine to Bionic Beaver (18.04 LTS) because Artful Aardvark (17.10) is no more supported and thus must be considered as outdated and insecure!
if you are unable to upgrade you can likely use the packages from Debian Stretch or if not working, try them from Xenial Xerus (16.04 LTS) or Bionic Beaver (18.04 LTS). BUT it is possible that you'll end sooner or later in dependency hell.
but this is just advised for testing and/or learning purpose and i would never use the machine in any production/real environment again before setting up from scratch.