Thanks to Jens W. Klein, there is an answer to that question on StackOverflow: Use the
Products.PlonePAS.events.UserLoggedOutEvent event (or, probably better:
He created a Plone documentation issue as well.
configure.zcml file I have now:
<include package="Products.PluggableAuthService" /> <subscriber for="Products.PluggableAuthService.interfaces.events.IUserLoggedOutEvent" handler=".events.after_logout"/>