Xenko 3D is now available as open source with the MIT permissive license with a completely free runtime and editor. The announcement was made on the Xenko blog with the release of Xenko 3.0.
In addition to the open source transition, Xenko 3.0 includes a new C# project system and video, hair, and skin rendering. You can view the full release notes here.
Silicon Studio no longer supports Xenko, but members of the Xenko team will continue to work on it independently with the community.