It sure seems like S3D future development is dead. But.
Nothing else out there that I have found, free or otherwise, can dual dual filament printing better. All the other slicers treat multiple filament as some side-job, with little attention paid. CURA seems particularly random in how it deals with dual-prints. I hear great things from Prusa slicer, but to be honest, the interface is clunky and difficult to drive, and I have yet to get a good print on any of my reprap printers using it. The Prusa slicer is tuned to Prusa printers and apparently works great there.
Anyway, I use S3D for most of my work because as its name states "Simplify", it surely does that! They nailed that part for a slicer.
So, S3D is apparently "undead" at this time, I for one, hope that it is resurrected, maybe by a competitor? And extended into the future. It is a very good base upon which to start, after all.
Robotics and sensors engineer
And lots of other stuff for fun.