The current implementations of ImageGraphicFactory and SVGGraphicFactory both use a cache internally and provide a resetCache() method. However, that method is not part of the interface – instead, GeoServer calls it directly in the WMSLifecycleHandler::onReset() method.
In order to support additional ExternalGraphicFactory implementations that also use a cache, there should be an additional interface (CachingExternalGraphicFactory?) that provides a general cache reset capability.
See discussion https://sourceforge.net/p/geoserver/mailman/message/35747439/