What a beautiful game and project. It's shocking that they just disappeared like this!
I'm experienced with online research, so I started making contact with people who might know what happened to them. Their public info is published here (from their public business):
A true mystery. Hopefully someone can clarify what happened. If something did happen to them, we might hope to make public their work on this game, so others can continue it in their honor and admiration.
UPDATE February 2019:
I recently started investigating again, for no special reason. I tried contacting a mutual college friend from years past. To my surprise, I got a response! It was curious and tentative, worrying if I might have bad intentions with my questions. I did my best to clarify about this game and community, and got a positive response.
Apparently, the lead developers have lost contact with some of their old friends and family in another country, and nobody is sure what's going on. He said his sister is friends with the developer's sister, and she can vouch they're alive and well. :-) So, the first conclusion is that Traverse has been [temporarily?] abandoned. The reasons must be not great, which would explain the lack of explanation.
Hey, life happens.
And then to my great surprise, the sister herself contacted me to ask about this. Again, suspicious of my motives. I explained it all with links. She told me more about their lack of information and that this is very likely just an incident. The assumption I'm making is some personal reason to drop the game project. No accident or deaths - nothing bad like that.
So with all of that, hopefully in the future the developers will come back and rejuvenate Traverse. And if not, maybe some explanation will be due. Either way, I'm happy to have resolved some of this fascinating online mystery, and to have shared it with you - fellow fans.