Update to Drupal 8.0.0 beta 14. For more information, see https://drupal.org/node/2544542

core/modules/system/templates/feed-icon.html.twig

@@ -5,14 +5,13 @@
  *
  * Available variables:
  * - url: An internal system path or a fully qualified external URL of the feed.
- * - icon: The rendered HTML (<img> tag) for the feed icon image.
  * - attributes: Remaining HTML attributes for the feed link.
  *   - title: A descriptive title of the feed link.
  *   - class: HTML classes to be applied to the feed link.
  *
- * @see template_preprocess_feed_icon()
- *
  * @ingroup themeable
  */
 #}
-<a href="{{ url }}"{{ attributes.addClass('feed-icon')|without('href') }}>{{ icon }}</a>
+<a href="{{ url }}"{{ attributes.addClass('feed-icon') }}>
+  {{ 'Subscribe to @title'|t({'@title': title}) }}
+</a>