Add Classic Classes Deprecation Guide

content/ember/v6/deprecate-classic-class-syntax.md

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+---
+title: "Deprecation: Classic Class Syntax"
+until: 7.0.0
+since: 6.5.0
+---
+
+With the introduction of native class syntax in JavaScript, Ember is deprecating its classic class syntax, which includes `extend`, `reopen`, and `reopenClass`. This guide will help you migrate your code to the modern syntax.
+
+### `extend`
+
+The `extend` method was used to create a new class that inherited from a parent class. This can be replaced with the `class` and `extends` keywords in native JavaScript.
+
+**Before:**
+
+```javascript
+import EmberObject from '@ember/object';
+
+const MyService = EmberObject.extend({
+  // ...
+});
+```
+
+**After:**
+
+```javascript
+import EmberObject from '@ember/object';
+
+class MyService extends EmberObject {
+  // ...
+}
+```
+
+### `reopen`
+
+The `reopen` method was used to modify an existing class by adding new properties and methods. This is a risky pattern because it modifies the class for all consumers, which can lead to unexpected behavior. There is no direct replacement for `reopen`. Instead, you should refactor your code to avoid modifying classes at runtime. If you must add functionality, consider using composition or creating a subclass.
+
+**Before:**
+
+```javascript
+import MyService from './my-service';
+
+MyService.reopen({
+  // ...
+});
+```
+
+**After:**
+
+There is no direct replacement for `reopen`. You should refactor your code to avoid this pattern. If you need to add functionality, consider creating a subclass:
+
+```javascript
+import MyService from './my-service';
+
+class ExtendedService extends MyService {
+  // ...
+}
+```
+
+### `reopenClass`
+
+The `reopenClass` method was used to add static properties and methods to a class. This can be replaced with the `static` keyword in native JavaScript.
+
+**Before:**
+
+```javascript
+import MyService from './my-service';
+
+MyService.reopenClass({
+  isMyService: true,
+});
+```
+
+**After:**
+
+```javascript
+import EmberObject from '@ember/object';
+
+class MyService extends EmberObject {
+  static isMyService = true;
+}
+```