The Summer of Code project was unsuccessful for personal reasons. The project was also never even close to high priority, because the daemon just isn't all that interesting beyond performing its one job of implementing the functional package management paradigm. That is to say, for a long time there was very little to be gained from a rewrite.

The most recent project has only just started and is spearheaded by Chris Baines, who has extensive experience with the build side due to his work on a (Guile-based) build coordinator used by one of the two major independent build farms.

I don't think the older branches contain much that needs to be ported; most of the work from past projects has in fact made it into Guix already and is used for other features --- just not in the daemon. If the Guile daemon project is successful, the daemon will incorporate many of these existing features for build isolation and coordination.