public List<ServerName> listDecommissionedRegionServers() {
    return this.serverManager.getDrainingServersList();
  }
List region servers marked as decommissioned (previously called 'draining') to not get regions
assigned to them.
@return List of decommissioned servers.
hbase-server/src/main/java/org/apache/hadoop/hbase/master/HMaster.java