Current text in https://oasis-tcs.github.io/odata-specs/odata-protocol/odata-protocol.html#UpsertanEntity says

Key and other non-updatable properties, as well as dependent properties that are not tied to key properties of the principal entity, MUST be ignored by the service in processing the Upsert request.

In the allowed case of single-valued non-containment navigation with navigation property binding the key property values cannot be ignored as they cannot always be derived from key values provided in the URL.