How do I unlink all contacts at once?

When you look deeply you will find that Contacts are neither stored on an iPhone nor on SIM. They are synced to an email account that you may have added and enabled Contacts. It could be Google, iCloud, AOL, Exchange, Outlook, Yahoo or any Other email client server.

When you enable Contacts in any of the email accounts they are displayed to you through the Contacts App. When you edit these contacts on an iPhone they are ultimately edited on the client-server.

Please see the pic below for a better understanding...

Apple Inc says; "We read all feedback carefully, but please note that we cannot respond to the comments you submit."

Feedback - Apple.com Website - Apple

Where are your Contacts stored?