Yes.
I’m not sure if one should put any resources on issues related to legacy saving. But if you guys have experience in legacy saving occurring all the time then - given that the GP is fundamentally differently handled in RH6 - I would suggest an automatic disabling of the GP when saving in the RH5 format.
… checking some more now…
I’m confused now 
I’m going through the Document Properties and Rhino Options in RH5 now and cannot find any reference to the Ground Plane there. The only place to handle this is when calling GroundPlane. But obviously there must be a setting in the .3dm format for Rhino 5 because else this wouldn’t carry over from RH6. So that must mean that the setting just is not exposed in the the Document Properties?
At any rate, if people need to legacy save it probably won’t be for the display features of RH5 but either just to get a hold of the geometry or because of legacy plug-ins. I would just disable the GP…