Quick update and big props to AaronPulver for pointing out my error. Here is version of the code which will allow you to move completed assignments to a separate feature layer (archived) .
# connect to org/portal
gis = GIS(orgURL, username, password)
project = workforce.Project(gis.content.search("type:'Workforce Project' 'name of WF Project'")[0])
#Archive Hosted Feature layer item ID (Note-The index "0" means the first layer in your feature service, so if you service has multiple layers, you may need to change that.)
completed_archive = gis.content.get("d25ed4b313bd499994b4727a6584a22213").layers[0]
#Query for Completed Assignments
completed_assignments = project.assignments.search(where= ' status=3 ' )
#Convert to Features
completed_features = [f.feature for f in completed_assignments]
# add features to layer
completed_archive.edit_features(adds=completed_features)
# delete assignments
project.assignments.batch_delete(completed_assignments)