But isn’t the purpose of Multi Object Editing to treat all selected objects as if they were one object in Edit Mode? So I think it is desired to propagate a change to all objects.
After playing around with the tool a bit I think “propagate to shapes” should be simply applied to each selected object individually. I’m not sure though, whether the operator should abort when at least one selected object does not have shape keys or whether it should ignore it (or print warning?) and propagate shapes for other selected objects with shape keys.
EDIT: here’s a diff to explain what I mean