I been testing and tracing the code and as far as I can determine ViewMutationStage is unused. Looking at StorageProxy::mutateMV if the MV mutation is on same endpoint as the base table endpoint it calls mutation.apply(writeCommitLog); which happens on MutationStage.
If the endpoint is a different endpoint it ends with call to asyncWriteBatchedMutations(wrappers, localDataCenter, Stage.VIEW_MUTATION); which in turn calls sendToHintedReplicas. But this method only uses the passed in stage if executing locally, which in case of the MV mutations is never true in my testing.
So it passes message with MUTATION_REQ which gets handled on the receiving node via MutationStage.
So at no point does the ViewMutationStage get used. I even tried to execute some of the View tests that check for pending tasks on this stage. Well there never even a completed task on that stage so these tests seem to wait on ViewMutationStage for no reason as best I can tell.
Even if the stage is used somewhere, its not used for all view mutations. So this stage is currently in my opinion code bloat and just confusing for the system operator.
An alternative solution to this ticket would be to implement VIEW_MUTATION_REQ etc and have everything MV mutation related run on the ViewMutationStage but that no easy change to make. See CounterMutation for example.