Hello @中垣 茂(ESM) ,
I understand that you were unable to delete a subnet because of serviceAssociationLinks/AppServiceLink and you already tried the steps mentioned here but it didn't help.
We offered you one-time free support as you requested, but you didn't have the permissions to create support requests in Azure portal, so you asked your service provider, and they created a support request for this issue.
The support team removed the orphaned link from the backend, and you were able to delete the subnet now.
Kindly let us know if you need further assistance on this issue.
Please "Accept the answer" if the information helped you. This will help us and others in the community as well.