No matter what they claim, it's really not supported in the sense most commercial oss projects are. We finally switched off after a minor version introduced a segfault when adding nodes in certain conditions, and the issue was unfixed after 5 months.
It exists but in terms of people using it or it being actively developed, it's dead as a doornail ever since Docker was more less forced to also support kubernetes and basically gave in to the reality that world + dog was opting for kubernetes instead of swarm.
They never really retired it but at this point it's a footnote in Docker releases.
I've not actually encountered it in the wild in four years or so and never in a production setup.
We are actually currently in the process of migrating from Docker Swarm to k8s and I am not 100% sure that's a good idea. We will see.