Add example module with a deprecation

web/modules/custom/example/example.info.yml

@@ -0,0 +1,7 @@
+name: Example module
+description: An example module.
+type: module
+core_version_requirement: ^8 || ^9
+dependencies:
+  - drupal:filter
+  - drupal:node

web/modules/custom/example/example.module

@@ -0,0 +1,17 @@
+<?php
+
+/**
+ * @file
+ * An example module.
+ */
+
+declare(strict_types=1);
+
+use Drupal\Core\Entity\EntityInterface;
+
+/**
+ * Implements hook_entity_view().
+ */
+function example_entity_view(array &$build, EntityInterface $entity): void {
+  drupal_set_message(t('This is a node.'));
+}

web/modules/custom/example/tests/src/Functional/DisplayMessageTest.php

@@ -0,0 +1,24 @@
+<?php
+
+namespace Drupal\Tests\example\Functional;
+
+use Drupal\Tests\BrowserTestBase;
+
+class DisplayMessageTest extends BrowserTestBase {
+
+  protected $defaultTheme = 'stable';
+
+  protected static $modules = ['filter', 'node', 'example'];
+
+  /** @test */
+  public function a_message_is_displayed_on_nodes(): void {
+    $this->drupalCreateContentType(['type' => 'page']);
+
+    $this->drupalCreateNode();
+
+    $this->drupalGet('node/1');
+
+    $this->assertText('This is a node.');
+  }
+
+}