This feature request is still in the works. No additional details to share at this time.
But the team is hard at work to ensure it is implemented in a fashion that will meet your expectations.
-WarrenDavid Costar commented
Channel level security is a significant gap that limits the full usage of Teams. Having a Teams site for an enterprise-wide department is effective, but not being able to limit specific artifacts and sensitive information to a few within that department continues to be an issue.