Unfortunately because of changing priorities, this feature was moved to the backlog. The team hopes to pick it up again in the near future.
Pinning the entire document library is supported today. Learn more here, https://support.microsoft.com/en-us/office/add-a-sharepoint-page-or-list-to-a-channel-in-teams-131edef1-455f-4c67-a8ce-efa2ebf25f0b.
We currently do not support pinning a specific folder, but we are considering this feature.
This feature request is working its way through the backlog queue. No new details are available yet.