Bug 967349 adds an API so that things using doorhangers (PopupNotifications) can simply add a "Learn More" link by specifying a URL (vs manually jamming bits into the DOM, or using <popupnotification> markup to add one). That patch removes the <label id="servicesInstall-learnmore-link" class="text-link"/> from the services, but keeps the otherwise-empty <popupnotification> due to a _bunch_ of tests doing |let panel = document.getElementById("servicesInstall-notification");|. These should be switched to PopupNotifications.getNotification(id,browser), but given the scale of the changes I didn't want to block on that. Once that's done we can remove the <popupnotification id="servicesInstall-notification">

deprecation in fx51

What does "deprecation" mean exactly? Can we now remove the <popupnotification> element from popup-notifications.inc without worrying about the 3 remaining tests or should we just remove the tests as well?

Most of the socialapi functionality was deprecated/removed, but share remains, and install notification also needs to remain. I suppose this should be reopened.

SocialAPI was removed from Firefox 57 and is no longer available in any current release.