use HTMLElement.dataset over getAttribute since all browsers GitLab supports implement it

app/assets/javascripts/subscription.js.es6

@@ -24,7 +24,7 @@
       button.classList.add('disabled');
       const isSubscribed = buttonSpan.innerHTML.trim().toLowerCase() !== 'subscribe';
-      const toggleActionUrl = this.containerElm.getAttribute('data-url');
+      const toggleActionUrl = this.containerElm.dataset.url;
       $.post(toggleActionUrl, () => {
         button.classList.remove('disabled');