In a couple of weeks I’m going to retire the server where announce.syncthing.net, the discovery server used by versions v0.11 and earlier, lives. This is a somewhat forced decision because the physical hardware is going away. It could be migrated to a virtual machine somewhere but that would take time and cost money, none of which I want to put into it.
v0.12.0 was released on Nov 5, 2015, roughly ten months ago. There are currently around 2.9% of users on v0.11 and 0.8% on v0.10. Some of these may certainly be installations that “just work” and no-one saw the need to upgrade. I suspect most are simply abandoned and forgotten about.