When a project is deleted, its directory JENKINS_HOME/jobs/<project> is wiped out. Workspace directories are not deleted if they have been defined outside the project directory (in global system configuration), which has been the default for a while. In multi-node environment, there can be several leftover workspaces (on all slaves where the project has been built).

There is an easy workaround for this. One can manually delete unwanted leftovers. It would be nice if Jenkins could do proper cleaning, or offer workspace deletion when a project is about to be deleted.