Microsoft plans to roll out private channel in Microsoft Teams in September 2019. This is one of the most requested features in Teams.
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.
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 2019.