How to change registration url user/register to /register in Drupal?

We can achieve this by using create a service and for custom service we required to create custom module.

Step 1: Create the mymodule.info.yml

name: 'My Module'
description: 'Custom Module'
package: Custom
type: module
core: 8.x

Step 2: Create the mymodule.services.yml file in your custom module

services:
mymodule.route_subscriber:
class: Drupal\mymodule\RouteSubscriber
arguments: []
tags:
- { name: event_subscriber }

Step:2 Create the src/RouteSubscriber.php Class :

<?php
namespace Drupal\mymodule;

use Drupal\Core\Routing\RouteSubscriberBase;
use Symfony\Component\Routing\RouteCollection;

/**
* Class RouteSubscriber
* @package Drupal\mymodule
*/
class RouteSubscriber extends RouteSubscriberBase {

/**
* @param RouteCollection $collection
*/
public function alterRoutes(RouteCollection $collection) {

if ($route = $collection->get('user.register')) { $route->setPath('/register'); }

}