Show "You are now signed out" message to users on logout

having same problem.

Drupal 7, I hack the function of user_logout() on user.pages.inc:

function user_logout() {
  global $user;

  watchdog('user', 'Session closed for %name.', array('%name' => $user->name));

  module_invoke_all('user_logout', $user);

  // Destroy the current session, and reset $user to the anonymous user.
  session_destroy();

  drupal_goto();
} 

Change drupal_goto()become to drupal_goto('', array('query' => array('logout' => '1')));. And then I make below function on my theme function file.

function MYTHEME_preprocess_page($var){
	if($_GET['logout'] == '1'){
		drupal_set_message('You are loged out.', 'info');
	}
}

However look dirty but it make it done.

Just before you call user_logout (or in hook_user_logout), set a cookie with the message you want to display

setcookie('my_message', t("Goodbye, have a nice evening!"), time() + (86400 * 30), "/");

... then in hook_init, display whatever is in the cookie and delete the cookie

function MYMODULE_init() {
  if(!empty($_COOKIE['my_message'])) {
    drupal_set_message($_COOKIE['my_message']);
    setcookie('my_message', '', time() - 3600);
  }
}

The easiest workaround is to simply do it like this:

module_load_include('inc', 'user', 'user.pages');
user_logout_current_user();
drupal_set_message('You have been logged out');
drupal_goto();

This does exactly the same thing as user_logout() and just sets the message after destroying the original session.
The message will still be stored in $_SESSION['messages'], but the user will no longer be authenticated.

This page is the first result on Google when I had problem about logout message, to help my future self I'll post here for Drupal 8.

Recreate the logout functionality. It's just calling user_logout() function.

For this, I create:

A method in SomeController

public function logout() {
  user_logout();
  return $this->redirect('<front>', [], ['query' => ['logout' => 1]]);
}

An event subcriber, subscribe to KernelEvents::REQUEST

$queryLogout = \Drupal::request()->query->get('logout');
if (!empty($queryLogout) && $queryLogout == 1) {
  drupal_set_message("You are logged out");
}

A router subcriber, altering user.logout route's controller

protected function alterRoutes(RouteCollection $collection) {

  // Change controller who handle logout.
  if ($routeLogout = $collection->get('user.logout')) {
    $routeLogout->setDefault('_controller', '\Drupal\my_module\Controller\SomeController::logout');
  }
}

The solution I was looking for !

At first I thought this was impossible in D7 without hacking core, as the session is destroyed and then the user is almost instantly redirected to the homepage. However, drupal_goto has an alter hook, This is during the new session so stuff can be added back in, or copied across in a global.

The below shows an example, the first 2 hooks, are all that is needed to pass data into the new session including calling drupal set message if needed.

/**
 * Implements hook_user_logout
 */
function my_module_user_logout($account) {
  $GLOBALS['flag_user_logout'] = $account->uid;
}

/**
 * Implements hook_drupal_goto_alter
 */
function my_module_drupal_goto_alter(&$path, &$options, &$http_response_code) {
  if (isset($GLOBALS['flag_user_logout'])) {
    $_SESSION['old_user_id'] = $GLOBALS['flag_user_logout'];
  }
}

/**
 * Implements hook_preprocess_page
 */
function my_module_preprocess_page(&$vars) {
  if (isset($_SESSION['old_user_id'])) {
    drupal_add_js('alert("bye user ' . $_SESSION['old_user_id'] . '");', 'inline');
    unset($_SESSION['old_user_id']);
  }
}