Update application-gateway-private-deployment.md

Added NOTE about subnet delegation requirement for Private Deployment Application Gateways (preview)
Since May 5, 2025 it was enforced on new and existing deployments

Author: BrandynR-MSFT

articles/application-gateway/application-gateway-private-deployment.md

@@ -187,6 +187,10 @@ The resource tag is cosmetic, and serves to confirm that the gateway has been pr
 
 Application Gateway Subnet is the subnet within the Virtual Network where the Application Gateway Resources will be deployed. In the Frontend Private Ip configuration, is important that this subnet can reach privately the resources that want to connect to your exposed app or site.
 
+> [!NOTE]
+> As of May 5, 2025 new and existing deployments of Private Application Gateway require Subnet Delegation to `Microsoft.Network/applicationGateways`.
+> Please follow [these steps](https://learn.microsoft.com/en-us/azure/virtual-network/manage-subnet-delegation?tabs=manage-subnet-delegation-portal) for configuring Subnet Delegation.
+
 ## Outbound Internet connectivity
 
 Application Gateway deployments that contain only a private frontend IP configuration (do not have a public IP frontend configuration associated to a request routing rule) aren't able to egress traffic destined to the Internet. This configuration affects communication to backend targets that are publicly accessible via the Internet.