@Override
  public void removedDistributedSystem(int remoteDsId) {
    removeCount++;
    List<Locator> locatorsConfigured = Locator.getLocators();
    Locator locator = locatorsConfigured.get(0);
    Map<Integer, Set<DistributionLocatorId>> allSiteMetaData =
        ((InternalLocator) locator).getLocatorMembershipListener().getAllLocatorsInfo();
    System.out.println("Removed : allSiteMetaData : " + allSiteMetaData);
  }
Please note that dynamic addition of the sender id to region is not yet available.
geode-junit/src/main/java/org/apache/geode/internal/cache/wan/MyDistributedSystemListener.java