Resolved as part of the Search 6 proof of concept groundwork.

The document identifier is not treated as a document field anymore; being a document field is an implementation detail in the backend. On the mapping side, a different type of bridge is used for identifiers: IdentifierBridge. The interface does not allow users to specify where to store the identifier, it just allows to specify how to turn the entity ID into a String to be used as a document ID, and vice-versa.