@Override
    protected void expireTombstone(Tombstone tombstone) {
      if (logger.isTraceEnabled(LogMarker.TOMBSTONE_VERBOSE)) {
        logger.trace(LogMarker.TOMBSTONE_VERBOSE, "adding expired tombstone {} to batch",
            tombstone);
      }
      synchronized (expiredTombstonesLock) {
        expiredTombstones.add(tombstone);
      }
    }
Returns true if keys needs to be tracked for clients registering interests on PR.
geode-core/src/main/java/org/apache/geode/internal/cache/TombstoneService.java