Issue
You did not delete your pipeline but it has disappeared from your dashboard.
Resolution
Access to pipelines are delegated by app access. If you do not have access to an app in the pipeline then the pipeline will not be visible to you. If all the apps in a pipeline are removed we cannot resolve who has access to it and the pipeline is automatically deleted.
For pipelines exclusively for review apps, it's recommended to keep a placeholder app in staging/production to avoid accidental deletion of the pipeline.