One-Stack Dev
The One-Stack Developer learned a stack, a technology, and simply stayed with it their whole career. This can be out of complacency, or can be an active decision to specialize in a desired or profitable niche. Or just life.
Some enterprise languages with a big ecosystem, such as Java, or Platform specific environments like .NET ( initially Windows only) or Objective C (Mostly Mac OS/iOS only) can lend themselves well to this. The vast amount of libraries, and their specialisation — in an ecosystem or platform — can be a wise career choice.
Unless you are prepared to give up something valuable you will never be able to truly change at all, because you’ll be forever in the control of things you can’t give up – Andy Law
Skills
Deep knowledge in one tech
The One-Stack Dev can perform stellarly in their programming language of choice. They know all the syntax and library by heart. On the other side, if not a conscious decision, they might be unable to think in other paradigms that their one language/stack does not support.