//absolute path from web root
require_once("extensions/redirectAfterLogout.php");

//wiki page to redirect to (adjust path and page title)
$wgPageToRedirectAfterLogout = "/path_to_mediawiki/index.php?title=yourpage";

<?php
//not called from the software -> show warning
if( ! defined( 'MEDIAWIKI' ) ) {
    echo( "This file is an extension to the MediaWiki software, and cannot be used standalone.\n" );
    die( 1 );
}
 
$wgExtensionCredits['other'][] = array(
    'name'=>'RedirectAfterLogout',
    'version'=> '1.0',
    'author'=>'Marcel Minke',
    'url'=>'https://www.mediawiki.org/wiki/Extension:RedirectAfterLogout',
    'description' => 'Redirect to a certain page after logout'
);
 
//wiki page to redirect to (adjust path and page title)
$wgPageToRedirectAfterLogout = "/path_to_mediawiki/index.php?title=yourpage"; 
 
$wgHooks['UserLogout'][] = 'redirectAfterLogout';
 
function redirectAfterLogout(&$user) {
        global $wgOut, $wgPageToRedirectAfterLogout;
        $wgOut->redirect( $wgPageToRedirectAfterLogout );        
        
        return true;
}