Release date: 11 Sep 2017.
New feature added: freeze deployment (pilot)
A new action "Freeze Deployment" was added under the "More" button. After you freeze a deployment, the source of your deployment points to a snapshot of your source org at the time of the freeze action. Changes in your live source org will no longer impact the content of your deployment. You cannot add or remove components after freezing a deployment. You can, however, unfreeze the deployment to continue building your change list.