Restbase's primary entrypoint is now through the text cluster, and the entrypoint in the parsoidcache cluster for the rest(base)?.wikimedia.org entrypoints shouldn't be necessary anymore. Need to verify lack of trailing dependencies/traffic first.
Are things still using the hostnames rest.wikimedia.org and/or restbase.wikimedia.org and/or restbase.eqiad.wikimedia.org, which map to the cache_parsoid cluster rather than through the normal text-cluster restbase entrypoints?
The problem with the redirect is it's complicated, because we still have this conflict between internal and external RB URLs due to the whole Host: header vs /hostname/ mess. It's possible, though.
Well, the other thing I can do to make this simple is just treat it like the legacy citoid/cxserver entrypoints: if it's one of the legacy restbase hostnames, just set the backend and (pass) all traffic.