I believe an engineer should learn to build things in the most pragmatic way possible using the best tool for the job. This requires breadth of experience across many areas, and a focus on delivery.
The idea of "Java engineer", "Scala engineer", "Golang engineer", etc. is so absurd to me. If you want to build a long lived career that will outlast tech fashion, learn many different tools and how to build software in different ways. Be known as someone who delivers, not an expert in language x or tool y.