Cleaves the glycosaminoglycan, dermatan sulfate. Eliminative cleavage of dermatan sulfate containing (1->4)-beta-D-hexosaminyl and (1->3)-beta-D-glucurosonyl or (1->3)-alpha-L-iduronosyl linkages to disaccharides containing 4-deoxy-beta-D-gluc-4-enuronosyl groups to yield a 4,5-unsaturated dermatan-sulfate disaccharide (DeltaUA-GalNAc-4S). Monomer. Belongs to the polysaccharide lyase 8 family.