diff --git a/typo3/sysext/backend/Classes/Template/DocumentTemplate.php b/typo3/sysext/backend/Classes/Template/DocumentTemplate.php
index 9d736a6e0465b3f324ce1c3f7fb3050230c75859..451ea2c53ca344a27a2665141a8db7d1e4c1a4a0 100644
--- a/typo3/sysext/backend/Classes/Template/DocumentTemplate.php
+++ b/typo3/sysext/backend/Classes/Template/DocumentTemplate.php
@@ -60,6 +60,7 @@ class DocumentTemplate
      * Additional header code for ExtJS. It will be included in document header and inserted in a Ext.onReady(function()
      *
      * @var string
+     * @deprecated since TYPO3 v8, will be removed in TYPO3 v9, use PageRenderers's JS methods to inject JavaScript on a backend page.
      */
     public $extJScode = '';
 
@@ -604,6 +605,7 @@ function jumpToUrl(URL) {
         }
 
         if ($this->extJScode) {
+            GeneralUtility::deprecationLog('The property DocumentTemplate->extJScode to add ExtJS-based onReadyCode is deprecated since TYPO3 v8, and will be removed in TYPO3 v9. Use the page renderer directly instead to add JavaScript code.');
             $this->pageRenderer->addExtOnReadyCode($this->extJScode);
         }
 
diff --git a/typo3/sysext/core/Documentation/Changelog/master/Deprecation-80512-DocumentTemplate-extJScodeProperty.rst b/typo3/sysext/core/Documentation/Changelog/master/Deprecation-80512-DocumentTemplate-extJScodeProperty.rst
new file mode 100644
index 0000000000000000000000000000000000000000..6331b549fbb1824b6eadcb658e117603e785fc4d
--- /dev/null
+++ b/typo3/sysext/core/Documentation/Changelog/master/Deprecation-80512-DocumentTemplate-extJScodeProperty.rst
@@ -0,0 +1,34 @@
+.. include:: ../../Includes.txt
+
+==========================================================
+Deprecation: #80512 - DocumentTemplate->extJScode property
+==========================================================
+
+See :issue:`80512`
+
+Description
+===========
+
+The property php:`DocumentTemplate->extJScode` to load ExtJS-specific code "onExtJSReady"
+has been marked as deprecated.
+
+
+Impact
+======
+
+If the property is filled and added to the response output, a deprecation warning will be triggered.
+
+
+Affected Installations
+======================
+
+Any installation with custom extensions using (or mis-using) this property to inject ExtJS-specific
+code.
+
+
+Migration
+=========
+
+Use the PageRenderer object directly to inject `addExtOnReadyCode` in a backend response.
+
+.. index:: Backend, PHP-API
\ No newline at end of file