I do not think it’s a bad idea to have the version tag on all blocks. Do we completely rule out that the storage schemes for the Polydata, unstructured or image data ever change in the future?
With the original proposal, one could imagine having a composite dataset of Unstructured grids where each block has version 1.0, since that was the file format version that included the unstructured grid description. Despite the entire file as a whole has version 3.0 (or whatever > 2.0). That indicate not just the type (polydata, unstructured, image) but also the storage layout version.
I also do not sew any gains in dropping it. We already have writers for various file formats that write the group with the data structures including the version tag, and not writing it will be more of a complication than just always writing it. Same thing with reading. If you just point any of today’s readers (Polydata, unstructured, image) towards a group in a compoiste, it will try to read the version tag. Implementing mechanisms to conditionally skip reading those few bytes would in my opinion not be with the potential gains…