I have also been looking for such a feature but have had no luck. This would be highly useful as the management at my company would like to add in their comments/annotations as they view the app via Server.
Would be great if any Qlik personnel could comment on whether this feature is in the pipeline for future releases!
This is possible using the standard text object in Qlik Sense, Community sheets (as opposed to the embedded sheets in a published application), and with a bit of creativity in security rules.
The issue is that there is no 'lock' on a sheet when a user is editing it. So to avoid unexpected outcomes it is best to only have one person allowed to edit the published sheet - ie. the person defined as the owner. So if you are say wanting to allow a department head to edit the page that contains their P&L, then make them owner of that page and setup security rules that allow the owner to edit a published sheet (including the objects on the sheet...ie. the text object).
If you wanted others to contribute to the sheet you could add an extension that allowed you to call the API to take control of the sheet by becoming the owner.
Otherwise Metric Insights provide a full featured platform for collaboration that integrates with Qlik.