It was morphed into SDL LiveContent back in 2014 I believe. At that point it was called "LiveContent Create" and you can see where it lies in the stack here: https://docs.sdl.com/LiveContent/content/en-US/SDL%20LiveContent%20full%20documentation-v1/GUID-55008675-34AC-45DA-9D19-F2D08A44D3F4
If you search on that site you're see numerous references to Xopus