COMDOX-1588: Revise backward-incompatible changes for 2.4.9

src/pages/development/backward-incompatible-changes/index.md

@@ -9,9 +9,9 @@ keywords:
 
 This page highlights backward-incompatible changes between Adobe Commerce and Magento Open Source releases that have a major impact and require detailed explanation and special instructions to ensure third-party modules continue working. High-level reference information for all backward-incompatible changes in each release is documented in the [reference](reference.md) section.
 
-## 2.4.9-beta1
+## 2.4.9
 
-The following major backward-incompatible changes were introduced in the 2.4.9-beta1 Adobe Commerce and Magento Open Source releases:
+The following major backward-incompatible changes were introduced in the 2.4.9 Adobe Commerce and Magento Open Source releases:
 
 * GraphQL alias limit validation
 * GraphQL query length validation
@@ -21,6 +21,21 @@ The following major backward-incompatible changes were introduced in the 2.4.9-b
 * Valkey 8.x CLI command
 * Zend_Cache replaced with symfony/cache
 
+### Admin Password Length Validation
+
+A new Admin configuration setting enables validation of minimum password length for admin users. Administrators can now set this requirement via **Stores > Configuration > Advanced > Admin > Security**. The validation is enforced during admin user create or update operations and configuration saves, with real-time frontend feedback for improved user experience.
+
+**Impact:**
+
+* Only custom or non-standard admin user creation flows are impacted if they do not comply with the new minimum password length requirement
+* No impact is expected on normal admin usage if passwords meet the configured minimum length
+* The default minimum length can be adjusted in configuration as needed
+* A constructor was added to the `Magento\User\Model\UserValidationRules.php` file to support this feature; EAT report shows zero affected modules
+
+**Action required:**
+
+No action is required for typical usage. To adjust the minimum password length, use the system configuration path `admin/security/minimum_password_length` in **Stores > Configuration > Advanced > Admin > Security**. \<!--AC-15249--\>
+
 ### GraphQL alias limit validation
 
 Validation was added to limit the number of aliases in GraphQL requests to ten. This limit prevents denial of service or resource degradation from excessive aliases. Previously, there was no limit on the number of aliases in a GraphQL request.\<!--AC-14701--\>