Added retirement notice for URL ping tests

Author: KennedyDenMSFT

articles/azure-monitor/app/availability-overview.md

@@ -14,6 +14,9 @@ You can set up availability tests for any HTTP or HTTPS endpoint that's accessib
 
 ## Types of tests
 
+> [!IMPORTANT]
+> On September 30, 2026, URL ping tests in Application Insights will be retired. Existing URL ping tests will be removed from your resources. Review the [pricing](/pricing/details/monitor/#pricing) for standard tests and [transition](https://aka.ms/availabilitytestmigration) to using them before September 30, 2026 to ensure you can continue to run single-step availability tests in your Application Insights resources.
+
 There are four types of availability tests:
 
 * [Standard test](availability-standard-tests.md): This single request test is similar to the URL ping test. It includes TLS/SSL certificate validity, proactive lifetime check, HTTP request verb (for example, `GET`, `HEAD`, or `POST`), custom headers, and custom data associated with your HTTP request.