hdiutil attach -shadow not working
Alas, it seems that either macOS has changed the format of .shadow files in a breaking way, or that my .shadow files are corrupted (hundreds of them) without any corruption to the DMGs themselves (hundreds of them, all with checksums that match). I'm going to guess it's the first option. Nonetheless, the usage message only shows when mounting with these specific shadow files. When I create new shadow files, the disk images mount just fine.