Actors do not appear to be unsubscribed from DistributedPubSub topics when they terminate.

PubsubUnsubscribeOnTerminateDemo.scala

package com.sandbox.pubsub

import akka.actor.{Actor, ActorLogging, ActorRef, ActorSystem, PoisonPill, Props}
import akka.cluster.pubsub.DistributedPubSub
import akka.cluster.pubsub.DistributedPubSubMediator._
import com.typesafe.config.ConfigFactory

object PubsubUnsubscribeOnTerminateDemo {
  def main(args: Array[String]): Unit = {
    val config = ConfigFactory parseString
      """
        | akka {
        |   actor.provider = "akka.cluster.ClusterActorRefProvider"
        |   cluster.pub-sub {
        |     send-to-dead-letters-when-no-subscribers = off
        |     remove-time-to-live = 100 ms
        |   }
        | }
      """.stripMargin
    val system = ActorSystem("PubsubTest", config)
    val pubsub = DistributedPubSub(system).mediator
    implicit val sender: ActorRef = system.deadLetters

    val actor1 = system.actorOf(Props(classOf[Subscriber]), "actor1")
    Thread sleep 1000
    
    pubsub ! Subscribe("TOPIC", actor1)
    pubsub ! Publish("TOPIC", "Ping")
    Thread sleep 1000

    actor1 ! PoisonPill
    Thread sleep 1000

    pubsub ! Publish("TOPIC", "Ping")

    // after running, the log reads:
    // [INFO] [akka.tcp://PubsubTest@xx.xx.xx.xx:xxxx/user/actor1] Ping
    // [INFO] [akka://PubsubTest/user/actor1] Message [java.lang.String] without sender to Actor[akka://PubsubTest/user/actor1#-1945572019] was not delivered. [1] dead letters encountered. This logging can be turned off or adjusted with configuration settings 'akka.log-dead-letters' and 'akka.log-dead-letters-during-shutdown'.
    // the DeadLetter indicates that the actor was not properly unsubscribed from the topic on termination
  }

  class Subscriber extends Actor with ActorLogging {
    override def receive: Receive = {
      case "Ping" => log.info("Ping")
    }
  }
}