Should the post_logout_redirect_uri parameter be provided to the end_session_endpoint, the OP needs to verify that the URI was previously registered to the requesting RP. A client_id is required to make that verification. Is the assumption that the OP will get this value from the id_token_hint? If true, that value is "recommended". Should the client_id be an explicit parameter? That's how I've currently got it implemented.
Issue #914 wontfix