Issue #3512254 by sayan_k_dutta, catch, smustgrave: Deprecate #type fieldgroup

core/lib/Drupal/Core/Installer/Form/SiteConfigureForm.php

@@ -106,6 +106,7 @@ public function buildForm(array $form, FormStateInterface $form_state) {
         ]));
     }
 
+    $form['#attached']['library'][] = 'core/drupal.fieldgroup';
     $form['#attached']['library'][] = 'system/drupal.system';
     // Add JavaScript time zone detection.
     $form['#attached']['library'][] = 'core/drupal.timezone';
@@ -114,7 +115,8 @@ public function buildForm(array $form, FormStateInterface $form_state) {
     $form['#attached']['drupalSettings']['copyFieldValue']['edit-site-mail'] = ['edit-account-mail'];
 
     $form['site_information'] = [
-      '#type' => 'fieldgroup',
+      '#type' => 'fieldset',
+      '#attributes' => ['class' => ['fieldgroup']],
       '#title' => $this->t('Site information'),
       '#access' => empty($install_state['config_install_path']),
     ];
@@ -146,7 +148,8 @@ public function buildForm(array $form, FormStateInterface $form_state) {
     }
 
     $form['admin_account'] = [
-      '#type' => 'fieldgroup',
+      '#type' => 'fieldset',
+      '#attributes' => ['class' => ['fieldgroup']],
       '#title' => $account_label,
     ];
     $form['admin_account']['account']['name'] = [
@@ -170,7 +173,8 @@ public function buildForm(array $form, FormStateInterface $form_state) {
     ];
 
     $form['regional_settings'] = [
-      '#type' => 'fieldgroup',
+      '#type' => 'fieldset',
+      '#attributes' => ['class' => ['fieldgroup']],
       '#title' => $this->t('Regional settings'),
       '#access' => empty($install_state['config_install_path']),
     ];
@@ -189,7 +193,8 @@ public function buildForm(array $form, FormStateInterface $form_state) {
     ];
 
     $form['update_notifications'] = [
-      '#type' => 'fieldgroup',
+      '#type' => 'fieldset',
+      '#attributes' => ['class' => ['fieldgroup']],
       '#title' => $this->t('Update notifications'),
       '#description' => $this->t('When checking for updates, your site automatically sends anonymous information to Drupal.org. See the <a href="@update-module-docs" target="_blank">Update module documentation</a> for details.', ['@update-module-docs' => 'https://www.drupal.org/node/178772']),
       '#access' => empty($install_state['config_install_path']),

core/lib/Drupal/Core/Render/Element/Fieldgroup.php

@@ -15,10 +15,23 @@
  *
  * @see \Drupal\Core\Render\Element\Fieldset
  * @see \Drupal\Core\Render\Element\Details
+ *
+ * @deprecated in drupal:11.2.0 and is removed from drupal:12.0.0.
+ *    Use \Drupal\Core\Render\Element\Fieldset instead.
+ *
+ * @see https://www.drupal.org/node/3515272
  */
 #[RenderElement('fieldgroup')]
 class Fieldgroup extends Fieldset {
 
+  /**
+   * {@inheritdoc}
+   */
+  public function __construct(array $configuration, $plugin_id, $plugin_definition) {
+    parent::__construct($configuration, $plugin_id, $plugin_definition);
+    @trigger_error('The ' . __CLASS__ . ' element is deprecated in drupal:11.2.0 and is removed from drupal:12.0.0. Use ' . Fieldset::class . ' instead. See https://www.drupal.org/node/3515272', E_USER_DEPRECATED);
+  }
+
   public function getInfo() {
     $info = parent::getInfo();
     $info['#attributes']['class'] = ['fieldgroup'];