Update to Drupal 8.0.3. For more information, see https://www.drupal.org/drupal-8.0.3-release-notes

core/modules/forum/forum.module

@@ -501,7 +501,12 @@ function template_preprocess_forums(&$variables) {
           }
 
           $row[] = array(
-            'data' => $topic->comment_count . $new_replies,
+            'data' => [
+              [
+                '#prefix' => $topic->comment_count,
+                '#markup' => $new_replies,
+              ],
+            ],
             'class' => array('forum__replies'),
           );
           $row[] = array(

core/modules/forum/src/Tests/ForumTest.php

@@ -514,6 +514,9 @@ class ForumTest extends WebTestBase {
     // Check that forum renders properly.
     $this->drupalGet("forum/{$this->forum['tid']}");
     $this->assertResponse(200);
+
+    // Verify there is no unintentional HTML tag escaping.
+    $this->assertNoEscaped('<', '');
   }
 
   /**