When you request to logout, all the Stack Exchange sites that will be affected are listed and there is a note at the bottom
If you're on a shared computer, remember to log out of your Open ID provider (Facebook, Google, Stack Exchange, etc.) as well.
but no explanation of how to do this is provided nor is a link to openid.stackexchange.com (or better the direct logout link).
There are dozens of questions about how to log out and expressing confusion about how OpenID works on Stack Exchange going back years (just glance at the linked and related headings on the right). This excludes general discussion of OpenID and other standards.
The sometimes almost 4 year old links below are among the more relevant even though some of the details have changed. Most of the "How do I logout?" questions or answers also apply.
Proposal 1. - Add an explicit link -- or better an option -- to also log out of OpenId for Stack Exchange. Ideally this would only be present if that credential is currently valid. More generally, the source of the credential actually used should be mentioned and an appropriate link provided.
Stack Exchange has changed (progressed!) several times. In a sense, this proposal is the logical culmination of the current logout page. An additional benefit is that it will also help train users in the correct way to use OpenID (or equivalent) specifically on Stack Exchange sites as well as more generally.