I found the old addresses here:
https://dnshistory.org
plan9.bell-labs.com was 204.178.31.16
and sources.cs.bell-labs.com was 204.178.31.32
Both gone too, its not just DNS.
I think it has fallen off its perch,
it is are pine-ing for the fjords,
it is an ex OS research group.
-Steve