unsubscribe_all_steamworkshop_items.js

// In chrome: Open the developer console with CTRL + J and paste the code bellow in the console

(function() {
  let elements = document.getElementsByClassName("btn_grey_black"); 
  for(var i = 0; i < elements.length; i++) {
    let element = elements[i],
        linkParts = element.getAttribute("href").split('\'');
    UnsubscribeItem(linkParts[1], linkParts[3]);
  }
})();