Diesel engine oil shortage predictions, will it happen?

Same thing. The DPF has nothing to do with the DEF, DEF is a part of the SCR system. The issue was dosing pump failures, so if there is no DEF available to inject, the EPA would likely put a temporary fix in place where the dealer network could flash each truck for no DEF until a certain date. The manufacturers would have to write the programming, and it would likely cost 1-2 hours of labor per truck for the flash (and the subsequent re-flash to original specs afterward).