The old (standalone) Swarm required a service registry so that all its managers can have the same view of the cluster state. When instantiating the old Swarm nodes, we had to specify the address of a service registry. However, if you take a look at setup instructions of the new Swarm (Swarm Mode introduced in Docker 1.12), you'll notice that we did not set up anything beyond Docker Engines. You will not find any mention of an external service registry or a key-value store.
Does that mean that Swarm does not need service discovery? Quite the contrary. The need for service discovery is as strong as ever, and Docker decided to incorporate it inside Docker Engine. It is bundled inside just as Swarm is. The internal process is, essentially, still very similar to the one used by the standalone Swarm, only with less moving parts. Docker Engine now acts as a Swarm manager, Swarm worker, and service registry.
The decision to bundle everything inside the engine...