Plinth beam is RCC beam which is provided to control differential settlement in a building or as a support for the wall. It is not required unless structurally necessary.
DPC is a 2-inch-thick PCC (1:2:4) layer with Bitumen coating and polythene sheet over it. This is necessary for moisture control. It is a necessary component of a building. However, if plinth beam is provided then DPC is not required as top of plinth beam can be coated with bitumen and PE sheet placed over it.