diff --git a/core/modules/forum/forum.module b/core/modules/forum/forum.module
index b4bf306ab8c34ba1b68742d49dc8267ee60cae44..1b15b3e899b3a056d84c4b0d2a7f005fc21a768f 100644
--- a/core/modules/forum/forum.module
+++ b/core/modules/forum/forum.module
@@ -683,26 +683,24 @@ function template_preprocess_forum_icon(&$variables) {
   $variables['hot_threshold'] = \Drupal::config('forum.settings')->get('topics.hot_threshold');
 
   if ($variables['num_posts'] > $variables['hot_threshold']) {
-    $icon_status_class = $variables['new_posts'] ? 'hot-new' : 'hot';
+    $variables['icon_status'] = $variables['new_posts'] ? 'hot-new' : 'hot';
     $variables['icon_title'] = $variables['new_posts'] ? t('Hot topic, new comments') : t('Hot topic');
   }
   else {
-    $icon_status_class = $variables['new_posts'] ? 'new' : 'default';
+    $variables['icon_status'] = $variables['new_posts'] ? 'new' : 'default';
     $variables['icon_title'] = $variables['new_posts'] ? t('New comments') : t('Normal topic');
   }
 
   if ($variables['comment_mode'] == CommentItemInterface::CLOSED || $variables['comment_mode'] == CommentItemInterface::HIDDEN) {
-    $icon_status_class = 'closed';
+    $variables['icon_status'] = 'closed';
     $variables['icon_title'] = t('Closed topic');
   }
 
   if ($variables['sticky'] == 1) {
-    $icon_status_class = 'sticky';
+    $variables['icon_status'] = 'sticky';
     $variables['icon_title'] = t('Sticky topic');
   }
 
-  $variables['attributes']['class'][] = 'icon';
-  $variables['attributes']['class'][] = 'topic-status-' . $icon_status_class;
   $variables['attributes']['title'] = $variables['icon_title'];
 }
 
diff --git a/core/modules/forum/templates/forum-icon.html.twig b/core/modules/forum/templates/forum-icon.html.twig
index f3447f8c072099222245fb21f181ed3024046253..b32b7346a15bee775a8910ac9369c4f15a252a8b 100644
--- a/core/modules/forum/templates/forum-icon.html.twig
+++ b/core/modules/forum/templates/forum-icon.html.twig
@@ -11,13 +11,20 @@
  * - icon_title: Text alternative for the forum icon, same as above.
  * - new_posts: '1' when this topic contains new posts, otherwise '0'.
  * - first_new: '1' when this is the first topic with new posts, otherwise '0'.
+ * - icon_status: Indicates which status icon should be used.
  *
  * @see template_preprocess_forum_icon()
  *
  * @ingroup themeable
  */
 #}
-<div{{ attributes }}>
+{%
+  set classes = [
+    'icon',
+    'topic-status-' ~ icon_status,
+  ]
+%}
+<div{{ attributes.addClass(classes) }}>
   {% if first_new -%}
     <a id="new"></a>
   {%- endif %}