According to section 3.7.3.2 of SAML Core spec :

If the session authority successfully terminates the principal's session with respect to itself, then it MUST respond to the original requester, if any, with a message containing a top-level status code of urn:oasis:names:tc:SAML:2.0:status:Success. If it cannot do so, then it MUST respond with a message containing a top-level status code indicating the error

The SAML IDP can (should but usually doesn't) respond to a <logoutRequest> message with a <LogoutResponse> message indicating a successful logout. We don't currently handle this case, and we should