I noticed some long-lived agents on ci.jenkins.io start to reach capacity on their disks. The major culprit seems to be these just-in-time allocated workspaces created by the parallel step.
From the file system of an agent in question:
I don't think these workspaces serve any reasonable purpose after the parallel() step has completed since you cannot browse or do anything else with them.
They should be removed to free up disk resources for other jobs on the agent.