Microsoft plans to gradually roll out private channel in Microsoft Teams in September late October (till early November) 2019. This is one of the most requested features in Teams. [UPDATED 17.09.2019]

Finally, this enables to manage and control access to channels in Teams. So only defined users can access a Teams channel within a specific Team.

Source: https://admin.microsoft.com/AdminPortal/Home#/MessageCenter [UPDATED]
Source: https://admin.microsoft.com/AdminPortal/Home#/MessageCenter [AUGUST 2019, obsolete]

Conclusion, opinion and summary

I’m looking forward to this feature update because this brings more manageability for team owners within a [Teams] team, if required. Especially, if you have a project team with external guests you might want to have an internal project-related conversation or data exchange, too. So, by secure private channels you have [soon] the option to define such “internal” channels for a project team within a [Teams] team. There’ll be no need to create an extra Team for this use case, just as one possible use case, e.g. you do not need to create two project Teams for [internal] and [external] collaboration. Instead you can utilize secure private channels and avoid creating duplicate Teams due to access-control needs for project contents and collaboration.

Right now ( 8th August 2019) I’m not aware of plans if the feature will come to existing [Teams] teams or only to newly created ones. I hope that it will apply to existing [Teams] teams but we’ll see in September October/November 2019. [UPDATED 17.09.2019]

Additional resources