Rebuild routes when config-based CSV importers are saved/deleted.

modules/core/import/modules/csv/farm_import_csv.services.yml

@@ -9,3 +9,8 @@ services:
     arguments: [ '@database', '@current_user', '@tempstore.private' ]
     tags:
       - { name: 'event_subscriber' }
+  farm_import_csv.config_subscriber:
+    class: Drupal\farm_import_csv\EventSubscriber\CsvMigrationConfigSubscriber
+    arguments: [ '@router.builder' ]
+    tags:
+      - { name: 'event_subscriber' }

modules/core/import/modules/csv/src/EventSubscriber/CsvMigrationConfigSubscriber.php

@@ -0,0 +1,54 @@
+<?php
+
+namespace Drupal\farm_import_csv\EventSubscriber;
+
+use Drupal\Core\Config\ConfigCrudEvent;
+use Drupal\Core\Config\ConfigEvents;
+use Drupal\Core\Routing\RouteBuilderInterface;
+use Symfony\Component\EventDispatcher\EventSubscriberInterface;
+
+/**
+ * Trigger a route rebuild when CSV importer configuration changes.
+ */
+class CsvMigrationConfigSubscriber implements EventSubscriberInterface {
+
+  /**
+   * The router builder.
+   *
+   * @var \Drupal\Core\Routing\RouteBuilderInterface
+   */
+  protected $routerBuilder;
+
+  /**
+   * Constructs the CsvMigrationConfigSubscriber.
+   *
+   * @param \Drupal\Core\Routing\RouteBuilderInterface $router_builder
+   *   The router builder service.
+   */
+  public function __construct(RouteBuilderInterface $router_builder) {
+    $this->routerBuilder = $router_builder;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function getSubscribedEvents() {
+    $events[ConfigEvents::SAVE][] = ['rebuildRouter'];
+    $events[ConfigEvents::DELETE][] = ['rebuildRouter'];
+    return $events;
+  }
+
+  /**
+   * Informs the router builder a rebuild is needed when necessary.
+   *
+   * @param \Drupal\Core\Config\ConfigCrudEvent $event
+   *   The Event to process.
+   */
+  public function rebuildRouter(ConfigCrudEvent $event) {
+    $config = $event->getConfig();
+    if (str_starts_with($config->getName(), 'migrate_plus.migration.')) {
+      $this->routerBuilder->setRebuildNeeded();
+    }
+  }
+
+}