I don't think it is productive to define "traditional" in a singular way...meaning we have to start with some definiton that we can all agree on. Again, the Oxford English Dictionary seems a standard to me.

And I would call attention to the phrase that defines "traditional" as being passed from one generation to the next. In that context, there is no chronological cut-off point. No arbitrary time frame when a style or implementation passes into historical obscurity. Only historical obscurity itself can impose that limit.

I, for one, will always regard the four yard box pleated kilt as traditional highland attire. I will always see the square bottomed notched lapel vest as traditional. And I'll add the TOS and the 18th century style buckle shoes, as well.

I believe that, given the evidence, even the flat cap could be included but I won't advocate for it.