In QGIS Server we have a new feature (that was developed alongside the context aware legend filter) which makes QGIS Server able to manage some new WMS query parameters for the GetLegendGraphics requests. These params are exactly the same syntax as in UMN Mapserver. These parameters let QGIS Server respond with context aware legend (i.e. only showing legend entries relevant to the current viewport. This is the mechanism used by the WMS legend inside QGIS Desktop. More details on this can be found in the QGIS Server Tutorial.