As discussed in the Hangout, there is no usage right now, but ISPN is expected add one in a custom sharding strategy. Sanne Grinovero expressed utter surprise that this is not the case yet as per ISPN master

We agreed to rename the method into something more general such as releaseWriteResources() which doesn't expose details of specific implementations and can rightfully be implemented as a no-op for index managers which don't need to release such resources.