Currently, private channels support connectors and tabs (except Stream, Planner, and Forms). Microsoft are still working on full apps support for private channels, including messaging extensions and bots. For more details about its description, you can refer to:
If the response is helpful, please click "Accept Answer" and upvote it.
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.