In a GetLegendGraphic request BBOX parameters are optional. If the new legend option ( GEOS-8114 Resolved ) legend_options=countMatched is used in a stand-alone GetLegendGraphic request the BBOX becomes mandatory. If BBOX is missing the GetLegendGraphic request fails with "GetMap requests must include a BBOX parameter".
While this breaks WMS standard, I propose to use the layers bbox as default if BBOX is not available in the GetLegendGraphic request. This will make this feature more robust and offers the possibility to show layer-overall counts in a legend graphic (not only depending on the recent requested map extend) alternatively by not sending a bbox param.