logoutMechanism is not documented on https://extensions.xwiki.org/xwiki/bin/view/Extension/OpenID%20Connect/OpenID%20Connect%20Authenticator/, so I guess it never really been a thing. I assume someone started working on it, never finished and committed that part along with other stuff by mistake. It's also not clear to me how you deduce from the little code there is that the intended value is the string "NONE".
