Actually, it looks like it's been officially removed from Neutron
since Ocata, when https://review.openstack.org/399320 merged (so
nearly 3.5 years already). As such it's just the code hosting which
needs to be retired, and there doesn't seem to be any governance
follow-up needed.
Thanks Jeremy, yes, that's correct, networking-calico has been a 'big tent' project but not 'Neutron stadium' for the last few years.
I'll take care to leave a good pointer in the README.rst, as you advised.
Best wishes,
Neil