I installed statusnotifier in addition now as recommended. Still some applications have issues. Now I cannot easily tell anymore which ones use a "tray" icon or which appear in the appindicator. Hexchat, RocketChat, Skype seem to work more or less fine (I think via tray) while - SpiderOak does not show an icon but has an active element which can be controlled (e.g. context menu) - Zoom does not appear anywhere when autostarted but appears (likely in statusnotifier) when (re)started afterwards. Since I have no idea how to debug if it's the application's fault or Xfce's this is probably a dead end. Anyway I wanted to post my feedback.