My hunch is that as long as elastic has been readily available, that elastic topped flashes have been used. When people talk about "traditional" with respect to garter ties, I think they're referring to the 18th and 19th centuries. The lack of availability of garter ties at modern highland games and many, if not most, kilt/scottish attire retailers would seem to be an indicator to me that garter ties are not considered de rigeur among most kilt wearers these days...and probably haven't been for several decades.