How to alter a route - Drupal 9

Sometimes you have to alter a route that is defined by a contrib module or Drupal core. Don't hack the module and change the *.routing.yml file. That is a very bad idea because the next time you update the module those changes will be gone.

The best way to alter a route in Drupal 9 is to use a Route subscriber. In the following example, I had to change the allowed auth mechanism for the JWT auth issuer route.

MY_MODULE.services.yml

services:
  MY_MODULE.route_subscriber:
    class: Drupal\MY_MODULE\Routing\RouteSubscriber
    tags:
      - { name: event_subscriber }

 src/Routing/RouteSubscriber.php

<?php

namespace Drupal\MY_MODULE\Routing;

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

class RouteSubscriber extends RouteSubscriberBase {

  /**
   * {@inheritdoc}
   */
  protected function alterRoutes(RouteCollection $collection) {
    if ($route = $collection->get('jwt_auth_issuer.jwt_auth_issuer_controller_generateToken')) {
      $route->setOption('_auth', ['basic_auth', 'email_basic_auth']);
    }
  }

}

Obviously, the $route object has a bunch of different useful methods like $route->setDefault() and $route->setRequirement(), so you can change anything you want.