Question 

A VMSS created by AKS exists, and the automatically generated Load Balancer (LB) during AKS creation is assigned to this VMSS. When attempting to delete the LB, it is indicated that deletion is not possible because it is in use by the VMSS. Is there a way to delete it?

If deletion is not possible, is there a way to reduce costs?
(If there is no traffic, will it not incur charges?)

Answer 

If you have created a basic AKS setup that already includes a Public Load Balancer, unfortunately, there is no way to remove it.

Defining the Load Balancer SKU can only be done when creating the AKS cluster. Once the AKS cluster is created, the Load Balancer SKU cannot be changed or deleted.[1]

As shown in the reference link, this configuration considers maintaining connectivity between the Control Plane and Agent nodes as well as backward compatibility. Therefore, unless the cluster was initially configured with "--outbound-type userDefinedRouting"[2], it is difficult to retroactively ensure compatibility while modifying the setup.

For Standard Load Balancers, it has been confirmed that an outbound rule for connectivity is automatically added, which incurs a base charge. Additionally, when outbound traffic is generated, charges apply based on the amount of data processed.[3]

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[1] Use a public Standard Load Balancer with Azure Kubernetes Service (AKS)
https://learn.microsoft.com/ko-kr/azure/aks/load-balancer-standard#limitations

[2] Control egress traffic for cluster nodes in Azure Kubernetes Service (AKS)
https://learn.microsoft.com/ko-kr/azure/aks/limit-egress-traffic

[3] Pricing – Load Balancer
Pricing—Load Balancer | Microsoft Azure