Notify WordPress menu update exceeding max_input_vars

menu-update-warning.php

<?php
/*
 * Plugin Name: Menu Update Warning
 * Plugin URI:
 * Description:
 * Version: 0.1
 * Author:
 * Author URI:
 */

add_action( 'admin_head-nav-menus.php', function() {
	if ( 'POST' !== $_SERVER['REQUEST_METHOD'] )
		return;

	add_action( 'admin_notices', function() {
		$items_sent = count( $_POST, 1 );
		$items_accepted = ini_get('max_input_vars');

		if ( $items_sent > $items_accepted ) {
			$notice = sprintf(
					__( 'Error: Menu update sent %d items while the server is configured to accept no more than %d items. Some of the menu items may have been lost!' ),
					$items_sent,
					$items_accepted
				);

			printf(
				'<div class="error">
					<p>%s</p>
				</div>',
				esc_html( $notice )
			);
		}
	} );
} );