HashMap getDestroyedSubregionSerialNumbers() {
    if (!isDestroyed) {
      throw new IllegalStateException(
          String.format(
              "Region %s must be destroyed before calling getDestroyedSubregionSerialNumbers",
              getFullPath()));
    }
    return destroyedSubregionSerialNumbers;
  }
Returns a map of subregions that were destroyed when this region was destroyed. Map contains
subregion full paths to SerialNumbers. Return is defined as HashMap because
DestroyRegionOperation will provide the map to DataSerializer.writeHashMap which requires
HashMap. Returns {@link #destroyedSubregionSerialNumbers}.

@return HashMap of subregions to SerialNumbers
@throws IllegalStateException if this region has not been destroyed
geode-core/src/main/java/org/apache/geode/internal/cache/LocalRegion.java