unsupported_feature· code· HTTP 501· scope bothA required protocol feature is not supported. It also carries the per-item rejection for a structurally valid Event whose producer class or wire feature has no admissible v1 form, such as an rfc9420.proposal decoded to an unsupported RFC 9420 sender class or Proposal type (mls-proposal-admission-registry.json); that case MUST NOT be reported as schema_violation. Dual-registered as a reason_code for the event_envelope scope (see reason_codes[]).