DOC Deprecate DataExtension and others

en/08_Changelogs/5.3.0.md

@@ -166,6 +166,7 @@ From 6.0 onwards, tokens will never be regenerated during session renewal, and t
 - Code for the CMS GraphQL `admin` schema which provided endpoints for the CMS has been deprecated and will be removed in CMS 6. Functionality which the GraphQL endpoints are currently responsible for will be replaced with regular Silverstripe controller endpoints instead. Extension hooks will be added to the new controller endpoints that return data to allow for customisation. Frontend schemas, such the `default` schema, will continue to work in CMS 6.
 - [`IPUtils`](api:SilverStripe\Control\Util\IPUtils) has been deprecated and its usage has been replaced with the `IPUtils` class from `symfony/http-foundation` which is now included as a composer dependency in `silverstripe/framework`.
 - Code in [`silverstripe/blog`](https://github.com/silverstripe/silverstripe-blog) which supported integration with [`silverstripe/widgets`](https://github.com/silverstripe/silverstripe-widgets) has been deprecated and will be removed in CMS 6.0.
+- [`DataExtension`](api:SilverStripe\ORM\DataExtension), [`SiteTreeExtension`](api:SilverStripe\CMS\Model\SiteTreeExtension), and [`LeftAndMainExtension`](api:SilverStripe\Admin\LeftAndMainExtension) have been deprecated and will be removed in CMS 6.0. If you subclass any of these classes, you should now subclass [`Extension`](api:SilverStripe\Core\Extension) directly instead.
 
 ## Bug fixes