diff -u b/core/modules/system/templates/admin-block-content.html.twig b/core/modules/system/templates/admin-block-content.html.twig
--- b/core/modules/system/templates/admin-block-content.html.twig
+++ b/core/modules/system/templates/admin-block-content.html.twig
@@ -16,12 +16,6 @@
  * @ingroup themeable
  */
 #}
-{%
-  set classes = [
-    'admin-list',
-    compact ? 'compact',
-  ]
-%}
 {% if content %}
   <dl>
     {% for item in content %}
reverted:
--- b/core/themes/classy/templates/admin-block-content.html.twig
+++ /dev/null
@@ -1,34 +0,0 @@
-{#
-/**
- * @file
- * Default theme implementation for the content of an administrative block.
- *
- * Available variables:
- * - content: A list containing information about the block. Each element
- *   of the array represents an administrative menu item, and must at least
- *   contain the keys 'title', 'link_path', and 'localized_options', which are
- *   passed to l(). A 'description' key may also be provided.
- * - attributes: HTML attributes to be added to the element.
- * - compact: Boolean indicating whether compact mode is turned on or not.
- *
- * @see template_preprocess_admin_block_content()
- *
- * @ingroup themeable
- */
-#}
-{%
-  set classes = [
-    'admin-list',
-    compact ? 'compact',
-  ]
-%}
-{% if content %}
-  <dl{{ attributes.addClass(classes) }}>
-    {% for item in content %}
-      <dt>{{ item.link }}</dt>
-      {% if item.description %}
-        <dd>{{ item.description }}</dd>
-      {% endif %}
-    {% endfor %}
-  </dl>
-{% endif %}
reverted:
--- b/core/themes/classy/templates/admin-block.html.twig
+++ /dev/null
@@ -1,26 +0,0 @@
-{#
-/**
- * @file
- * Default theme implementation for an administrative block.
- *
- * Available variables:
- * - block: An array of information about the block, including:
- *   - show: A flag indicating if the block should be displayed.
- *   - title: The block title.
- *   - content: (optional) The content of the block.
- *   - description: (optional) A description of the block.
- *     (Description should only be output if content is not available).
- *
- * @ingroup themeable
- */
-#}
-<div class="admin-panel">
-  {% if block.title %}
-    <h3>{{ block.title }}</h3>
-  {% endif %}
-  {% if block.content %}
-    <div class="body">{{ block.content }}</div>
-  {% elseif block.description %}
-    <div class="description">{{ block.description }}</div>
-  {% endif %}
-</div>
reverted:
--- b/core/themes/classy/templates/admin-page.html.twig
+++ /dev/null
@@ -1,26 +0,0 @@
-{#
-/**
- * @file
- * Default theme implementation for an administrative page.
- *
- * Available variables:
- * - system_compact_link: Themed link to toggle compact view.
- * - containers: An list of administrative blocks keyed by position: left or
- *   right. Contains:
- *   - blocks: A list of blocks within a container.
- *
- * @see template_preprocess_admin_page()
- *
- * @ingroup themeable
- */
-#}
-<div class="clearfix">
-  {{ system_compact_link }}
-  {% for container in containers %}
-    <div class="layout-column half">
-      {% for block in container.blocks %}
-        {{ block }}
-      {% endfor %}
-    </div>
-  {% endfor %}
-</div>