The current implementation of Shibboleth IDP does not support the back-channel logout functionality. Some specifications require that a logout happen over SOAP instead of front-channel SAML SLO flow.
The Shibboleth IDP is implementing the functionality in their upcoming release and integrating that release would allow Gluu to leverage this feature
https://issues.shibboleth.net/jira/browse/IDP-964
This is a future customer requirement for us, as it to be paralell with OIDC back channel (Issue #70).