In Jenkins Pipelines one can use shared libraries. With them a additional directory is created:

Usual-Workspace: /var/lib/jenkins/workspace/job-id/
Additional SharedLibrary-Folder: /var/lib/jenkins/workspace/job-id@tmp

Seems as if there the shared library sources are stored in case one want s to rerun the build.

In order to completely wipe out old build data, a option to delete this @tmp folder could be introduced.