By far the #1 committer and founding engineer of Swift for Tensorflow also recently left the project: http://rxwei.me/about/ . From the committer list, it looks like Dan Zheng is the only individual who is actively working on it, and he's was an intern until he formally started January 2019. I don't know the ins and outs of Google culture, but it looks at least like every senior engineer has moved onto something else.