Welcome to the Microsoft Q&A Platform. Thank you for reaching out & I hope you are doing well.
I understand that you are facing issues with abandoned Service Association Link preventing subnet deletion.
We have offered you an one-time free support to delete this SAL from the backend.
Kindly let us know if this helps or you need further assistance on this issue.
Please don’t forget to close the thread by clicking "Accept the answer" once you are able to delete the resource with the help of support, as this can be beneficial to other community members.