@katja I believe this is no longer a limitation on Microsoft’s end. They published documentation (last updated 6/23/2019) on how to implement pinnable task panes in Outlook: Implement a pinnable task pane in an Outlook add-in - Office Add-ins | Microsoft Learn
There are also at least two other duplicate “idea” requests for this same feature, not to mention quite a few other general forum discussions/requests:
Is there any way we can get this feature prioritized or at least some sort of confirmation of “we’re working on it”?
Any update would be greatly appreciated.