One question we've circled around is how a client can know that a manifest it pushed containing a subject field was processed by the registry, to know whether the client should push an index to the subject's fallback tag.

One solution would be for the registry to respond to a POST of a manifest with subject with a response header indicating that it's 1.1-compliant and is going to update the referrers response for you.

Something like X-OCI-Subject-Processed -- better names welcome.

Related: https://github.com/opencontainers/distribution-spec/blob/main/spec.md#unavailable-referrers-api

edit: This was @jonjohnsonjr 's idea