noblogredirect.txt

/**
 * If registration for new network sites is disabled,
 * the NOBLOGREDIRECT constant redirects to a specific URL
 * if a user tries to access a nonexisting network site.
 */
 
// add this to wp-config.php and adjust the URL
define( 'NOBLOGREDIRECT', 'https://example.com' );

/**
 * add this to mu-plugin to keep normal 404 errors 
 * working instead of redirecting to the above specified URL
 * @source: https://wordpress.org/support/topic/404-page-not-working-after-setting-noblogredirect/#post-4844131
 */
remove_action( 'template_redirect', 'maybe_redirect_404' );