It's not a decision taken lightly but, for a number of reasons, we're confident that it's the right move.
Firstly: interest in the Runtime SDK for macOS has been low. By freeing up team members from maintaining it (including not just the ArcGIS Framework, but also the Samples App, guide documentation, etc.), we'll be able to implement improvements across the entire Runtime SDK family more effectively.
Secondly: the decision was made easier given that developers targeting macOS as a platform still have the option of using the ArcGIS Runtime SDK for Java or ArcGIS Runtime SDK for Qt. If you are considering developing Runtime apps targeting macOS, we recommend you investigate those.
If you have questions about this deprecation, feel free to use the comments section below or, if you're coming to the Developer Summit in Palm Springs, come and see us at the developer island.