When ArcGIS Portal security is configured to use Windows Active Directory, (see config here), the "Sign Out" or "log out" link is hidden. Below image shows the UI and the html:
Below image shows the html when the "Sign Out" link is visible:
The ability to Sign Out of a site is a security issue and is a requirement for most organizations.
We need either an upgrade safe customization pattern to inject a "Sign Out" link, or a configuration file to change the visibilty and a custom endpoint or URL to log out from. The first option would just need to be in the help documents and the second option would be an additional configuration file which is here.
Either option just needs to result in inserting a custom endpoint or logout link URL with a return URL parameter.
It would be our responsibility to develop the custom endpoint which would need to expire the website's authentication cookie and then the esri_auth cookie to complete the log out.