It is clearly mentioned in Microsoft Documentation about private channels, that private channels can't be converted to standard channels and vice versa.
Refer: https://learn.microsoft.com/en-us/MicrosoftTeams/private-channels
Also private channels do not support planner.
https://learn.microsoft.com/en-us/MicrosoftTeams/private-channels
As per the above link the support may come in the near future, however it is not availble now.
Currently, private channels support connectors and tabs (except Stream, Planner, and Forms). We're working on full apps support for private channels, including messaging extensions and bots.
@Johannes Heinzelmann - Could you please accept the answer for more visibility?
@Johannes Heinzelmann
Haven't received your message for a long time.
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