add error code 451 - https://tools.ietf.org/html/rfc7725#section-3

src/Network/Exception/UnavailableForLegalReasonsException.php

@@ -0,0 +1,35 @@
+<?php
+/**
+ * CakePHP(tm) : Rapid Development Framework (http://cakephp.org)
+ * Copyright (c) Cake Software Foundation, Inc. (http://cakefoundation.org)
+ *
+ * Licensed under The MIT License
+ * Redistributions of files must retain the above copyright notice.
+ *
+ * @copyright     Copyright (c) Cake Software Foundation, Inc. (http://cakefoundation.org)
+ * @since         3.2.12
+ * @license       http://www.opensource.org/licenses/mit-license.php MIT License
+ */
+namespace Cake\Network\Exception;
+
+/**
+ * Represents an HTTP 451 error.
+ *
+ */
+class UnavailableForLegalReasonsException extends HttpException
+{
+
+    /**
+     * Constructor
+     *
+     * @param string|null $message If no message is given 'Unavailable For Legal Reasons' will be the message
+     * @param int $code Status code, defaults to 451
+     */
+    public function __construct($message = null, $code = 451)
+    {
+        if (empty($message)) {
+            $message = 'Unavailable For Legal Reasons';
+        }
+        parent::__construct($message, $code);
+    }
+}

src/Network/Response.php

@@ -73,6 +73,7 @@ class Response
         417 => 'Expectation Failed',
         422 => 'Unprocessable Entity',
         429 => 'Too Many Requests',
+        451 => 'Unavailable For Legal Reasons',
         500 => 'Internal Server Error',
         501 => 'Not Implemented',
         502 => 'Bad Gateway',