It's true that OSDD-1A and OSDD-1B are not differentiated in the DSM V, but this differentiation is used by many clinicians and researchers. This is similar to how Complex PTSD is used, even though it is not in the DSM V: it is used because it is useful.

From what I understand, there was a push from dissociation and trauma specialists to include Complex PTSD in the DSM V, and to create one category of traumatic stress disorders including the dissociative disorders and the PTSDs. This didn't happen apparently because the political implications for psychology would be too large at this time. Personally I believe dissociative disorders are under-researched and that OSDD could be broken down into different diagnoses if more were understood about dissociative disorders. The diagnostic criteria are afterall supposed to be useful, and what is considered useful changes as we learn more.