@Override
    protected Event doExecuteInternal(final RequestContext requestContext) throws Exception {
        val terminateSession = FunctionUtils.doIf(logoutProperties.isConfirmLogout(),
                () -> isLogoutRequestConfirmed(requestContext),
                () -> Boolean.TRUE)
            .get();

        if (terminateSession) {
            return terminate(requestContext);
        }
        return eventFactorySupport.event(this, CasWebflowConstants.STATE_ID_WARN);
    }
Parameter to indicate logout request is confirmed.
support/cas-server-support-actions-core/src/main/java/org/apereo/cas/web/flow/logout/TerminateSessionAction.java