Hierarchical names are complex and their applicability in jBPM 3 was quite limited. There is no need to support them in jBPM 4. The only plausible use case for them is the group secret section of the group activity chapter in the developers guide. It is worth noting that the secret feature described there goes against BPMN. If this feature is not based on community demand, has questionable semantics and requires extraneous activity naming restrictions, then it should be dismissed.
In sum, slashes in activity names should always be allowed without requiring a configuration switch. Note that this does not break backwards compatibility because all existing processes without slashes in their activity names will still be accepted.