Repro:

• Set up Google login plugin
• Login with Google account
• Click "logout"
• Get new page: "You have successfully logged out."
• 1) The login link at top right (https://myjenkins.foo/securityRealm/#) is now a 404
• 2) Open new tab with Jenkins. Am still logged in.

Since I logged in with my G account, I have been unable to logout of Jenkins, even after restarting and upgrading Jenkins.

There was a thread on this previously, but many of the posts contained confusing information about logging out of Google first: however, if this is plugin is implemented in the idiomatic way, this should not be necessary.