The class BinnedWcs is defined in meas_algorithms, but is not used anywhere. The class has no doc string so I'm not sure what it was intended for. It inherits from afw::image::Wcs but does not implement some of its methods and is not persistable.
I would like to get rid of it as part of the Wcs->SkyWcs change. If we ever do want to construct a binned WCS from an afw::geom::SkyWcs it would be simpler to construct a SkyWcs that adds a binning transform internally, thus producing a normal Wcs, thus offering persistence and all other SkyWcs methods, but no memory of the unbinned Wcs.