Are you sure this always works?

Well, it is a good question. But it is true for the previous implementation too.

.let { key ->
  Observable.fromIterable(cachedEvents[key])
  ...
}

RxJava2 version does not have @NonNull annotation, so Kotlin compiler allows passing a nullable value here, while it will still throw an exception in runtime.

RxJava3 version now has @NonNull annotation, so Kotlin compiler is more strict here.

I doubt it worked fine in the previous version as well. Because the result of the subscription is not used anywhere, hence there’s no manual unsubscribe implemented.