Planner for Private Channels

You can assign people of private channels tasks just fine. Private channel members are members of the Team so you can assign them tasks. The only thing you cannot do is have a "private" plan.

This is going to probably be awhile if at all since they have to rearchitect how Planner works to hide individual plans based on channels.

Anyway, you can use a website tab in a private channel to link straight to another plan if you want a link there, but you can most def. still assign tasks to anyone in the Team. If you need a private plan in general, then you'll have to have a Plan created on it's own, then a website tab linked to it and invite everyone in the channel, but at this point you might as well just create a new private Team for these people in the channel instead.