I’m pleased to announce we have began the initial phase of Safari extension testing, with a small group of users (some of you that have contributed to this thread have been contacted by us and by Apple via email already).
Our beta Safari extension only works in the new Paperpile. If you would like to be added to the next phase of testing, first make sure to sign up for the new Paperpile here. Once you have signed up, please activate the new Paperpile in Chrome by going to Settings > Feature Preview, wait for the sync to finish, and then open the new Paperpile web app in Safari. Go to Settings > Browser Extension to request access to the private beta.