Apps like Planner not available in private channel
Planner app works perfectly fine in standard channels of a Team. However, in any private channel (members are subset of Team), adding a new tab does not give option to add Planner app. There are many instances when a sub-team will be working on a special project and corresponding private channel will need its own planner.
Dean King commented
Second this. Would be nice to have a Planner tab in our private channels. The only other option would be to host the planner in the general channel which means everyone would then be able to see tasks intended for the private channel.
I'm assuming this fix is coming as private channels have only just rolled out. Maybe they just missed it.