Jenkins adds the plugin names and versions to the XML configuration, e.g. like plugin="email@example.com". Analyzing config.xml files it is possible to see what pluins are used by a job.
With workflow this is not possible, since the information what plugin is used, is in the script file. It is probably difficult to analyze the script statically and extract plugin information.
However, when a script is executed, workflow has the needed information. The steps are already logged to builds/<job-id>/workflow/<step-id>.xml.
For a git checkout, via the generic SCM API it logs:
Each step node contains also the used plugin(s) and version. In the above case it needs to log workflow-scm-step and git.