Deprecate public properties that have accessor methods.

This is only the beginning of the public property deprecations. More
properties will be deprecated as new accessor methods are introduced.

Refs #9325

src/Network/Request.php

@@ -23,10 +23,6 @@
 /**
  * A class that helps wrap Request information and particulars about a single request.
  * Provides methods commonly used to introspect on the request headers and request body.
- *
- * Has both an Array and Object interface. You can access framework parameters using indexes:
- *
- * `$request['controller']` or `$request->controller`.
  */
 class Request implements ArrayAccess
 {
@@ -35,6 +31,7 @@ class Request implements ArrayAccess
      * Array of parameters parsed from the URL.
      *
      * @var array
+     * @deprecated 3.4.0 This public property will be removed in 4.0.0. Use param() instead.
      */
     public $params = [
         'plugin' => null,
@@ -50,20 +47,23 @@ class Request implements ArrayAccess
      * data.
      *
      * @var array
+     * @deprecated 3.4.0 This public property will be removed in 4.0.0. Use data() instead.
      */
     public $data = [];
 
     /**
      * Array of querystring arguments
      *
      * @var array
+     * @deprecated 3.4.0 This public property will be removed in 4.0.0. Use query() instead.
      */
     public $query = [];
 
     /**
      * Array of cookie data.
      *
      * @var array
+     * @deprecated 3.4.0 This public property will be removed in 4.0.0. Use cookie() instead.
      */
     public $cookies = [];
 
@@ -597,6 +597,8 @@ public function __call($name, $params)
      *
      * @param string $name The property being accessed.
      * @return mixed Either the value of the parameter or null.
+     * @deprecated 3.4.0 Accessing routing parameters through __get will removed in 4.0.0.
+     *   Use param() instead.
      */
     public function __get($name)
     {
@@ -613,6 +615,8 @@ public function __get($name)
      *
      * @param string $name The property being accessed.
      * @return bool Existence
+     * @deprecated 3.4.0 Accessing routing parameters through __isset will removed in 4.0.0.
+     *   Use param() instead.
      */
     public function __isset($name)
     {