Release pipelines are in beta, and are available only to certain customers with an Enterprise or Guardian plan. Aspects of this feature may change without notice or become deprecated in favor of other release management features.
This topic explains how to delete release pipelines.
You can only delete release pipelines that do not have any active releases. If there are any flags in any phase of a release pipeline, you cannot delete the pipeline.
To delete a release pipeline:

You can also use the REST API: Delete release pipeline