Deleting data in Streak is irreversible from within Streak.
While possible to restore both boxes and pipelines by reaching out to the support team, the process isn't entirely foolproof and your most recent changes before deletion might be lost.
If you've created a pipeline accidentally or need to remove an old pipeline that no longer serves you, you can do so from the caret menu when hovering over the pipeline's name in the left navigation menu:
If the pipeline is empty, you'll be prompted to confirm. If the pipeline still has boxes in it, you'll be prompted to either Hide the pipeline instead, or first delete or move those boxes and then delete the pipeline. The GIF below shows how to delete boxes:
Note: In order to delete a pipeline you will need to be an admin on the pipeline. This article will explain the different pipeline permission levels. Pipeline permissions are separate from Team permissions.