The configuration documentation is incorrect or unclear that a default shutdown hook would be used if you use the configuration <shutdownHook/>.
If you don't provide a class like <shutdownHook class="ch.qos.logback.core.hook.DelayingShutdownHook"/> You will receive an error message like the following:
ERROR in ch.qos.logback.core.joran.action.ShutdownHookAction - Missing class name for shutdown hook. Near [shutdownHook] line xx
Please either change the documentation to state a class is a required attribute or you change ShutdownHookAction to use a default of no class attribute is given.