For the DeckSurf tooling (CLI and plugins), refer to the DeckSurf repository.
The SDK is used to manage Stream Deck devices and create plugins for the DeckSurf tools. It is completely independent of the Elgato software and/or libraries and can be used as a completely standalone library.
The SDK in its current implementation has a number of dependencies on Windows APIs, therefore will only work on Windows. In future releases, I am thinking of a way to rip out native components and separate them in their own package, allowing the SDK to be fully cross-platform.