diff --git a/code/LeftAndMainExtension.php b/code/LeftAndMainExtension.php
index 62835f1e4..9effb2106 100644
--- a/code/LeftAndMainExtension.php
+++ b/code/LeftAndMainExtension.php
@@ -3,15 +3,28 @@
 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()
     {