As a practical matter, the sync repo should not be in boostorg until it has a master branch. I discovered this anomaly by trying to check out every repo in the master branch with this git command:
iirc there used to be libraries in the past, which had been in trunk, but not in release.
Irrespective of whether a Boost.Synch spin-out is a good idea (sounds like it, but I don't know the details), any new library added to Boost, but not yet released will be in develop but not master (or at least master will be empty). Maybe the issue here is a transitional one due to the conversion - shouldn't synch be in master, albeit the directory will be empty? Perhaps creating an empty master branch would be a temporary fix - or would that mess up the release procedure? Whatever we need to figure out the right thing to do here so we know what happens when new libs are added, John.