I was looking at something else and ran across https://www.openstreetmap.org/changeset/11803832 which looks like an old import of building data. Does anyone know where the documentation for this is, or what source it was from?
Many of the changeset ways were replaced with newer, but those who remained original seem not to match any commercial source (check Bulmaps and Bgmaps) nor the Sofia cadastre. Probably this is unique data user ils ceated by himself. You’d better ask him about the data source.