diff --git a/code/CMSMenu.php b/code/CMSMenu.php
index 0318dda0a..b1be43881 100644
--- a/code/CMSMenu.php
+++ b/code/CMSMenu.php
@@ -27,7 +27,7 @@
  * configuration will also include a 'help' link to the SilverStripe user
  * documentation.
  *
- * Additional CMSMenu items can be added through {@link LeftAndMainExtension::init()}
+ * Additional CMSMenu items can be added through {@link Extension::init()}
  * extensions added to {@link LeftAndMain}.
  */
 class CMSMenu implements IteratorAggregate, i18nEntityProvider
diff --git a/code/CMSMenuItem.php b/code/CMSMenuItem.php
index c1852245f..89673e76a 100644
--- a/code/CMSMenuItem.php
+++ b/code/CMSMenuItem.php
@@ -10,7 +10,7 @@
 /**
  * A simple CMS menu item.
  *
- * Items can be added to the menu through custom {@link LeftAndMainExtension}
+ * Items can be added to the menu through custom {@link Extension}
  * classes and {@link CMSMenu}.
  *
  * @see CMSMenu
diff --git a/code/LeftAndMain.php b/code/LeftAndMain.php
index bb4b9853e..200ac3f80 100644
--- a/code/LeftAndMain.php
+++ b/code/LeftAndMain.php
@@ -611,8 +611,6 @@ public static function getRequiredPermissions()
     }
 
     /**
-     * @uses LeftAndMainExtension->init()
-     * @uses LeftAndMainExtension->accessedCMS()
      * @uses CMSMenu
      */
     protected function init()
diff --git a/code/LeftAndMainExtension.php b/code/LeftAndMainExtension.php
deleted file mode 100644
index 4a2ae8310..000000000
--- a/code/LeftAndMainExtension.php
+++ /dev/null
@@ -1,40 +0,0 @@
-<?php
-
-namespace SilverStripe\Admin;
-
-use SilverStripe\Core\Extension;
-use SilverStripe\Dev\Deprecation;
-
-/**
- * Plug-ins for additional functionality in your LeftAndMain classes.
- *
- * @template T of LeftAndMain
- * @extends Extension<T>
- * @deprecated 2.3.0 Subclass SilverStripe\Core\Extension\Extension instead
- */
-abstract class LeftAndMainExtension extends Extension
-{
-    public function __construct()
-    {
-        // Wrapping with Deprecation::withNoReplacement() to avoid triggering deprecation notices
-        // as we are unable to update existing subclasses of this class until a new major
-        // unless we add in the pointless empty methods that are in this class
-        Deprecation::withNoReplacement(function () {
-            $class = Extension::class;
-            Deprecation::notice('2.3.0', "Subclass $class instead", Deprecation::SCOPE_CLASS);
-        });
-        parent::__construct();
-    }
-
-    public function init()
-    {
-    }
-
-    protected function accessedCMS()
-    {
-    }
-
-    protected function augmentNewSiteTreeItem(&$item)
-    {
-    }
-}
diff --git a/tests/php/CMSProfileControllerTest/TestExtension.php b/tests/php/CMSProfileControllerTest/TestExtension.php
index be6f1347f..f2a069dd0 100644
--- a/tests/php/CMSProfileControllerTest/TestExtension.php
+++ b/tests/php/CMSProfileControllerTest/TestExtension.php
@@ -2,9 +2,9 @@
 
 namespace SilverStripe\Admin\Tests\CMSProfileControllerTest;
 
-use SilverStripe\ORM\DataExtension;
+use SilverStripe\Core\Extension;
 
-class TestExtension extends DataExtension
+class TestExtension extends Extension
 {
     public function canEdit($member = null)
     {