No, it was an Elefront Parameter data type (from the EF Text component). I had internalized that data in a native Data component, but it retained the data-type defined by the Elefront library, so Shapediver flagged it as unsupported.
As I understand it, Elefront Properties are merely Elefront’s name for User-Defined Keys and Values.
However, since User-Defined Keys and Values are only associated with referenced Rhino geometry, not Grasshopper representations of geometry, I don’t see how these could be used in Shapediver. When internalized, I think referenced Rhino geometry loses its attributes (gets converted to a Grasshopper data type instead)