I'm surprise that I had not inserted my 2 cents in that previous thread, but now I will:
I know the subject of this was a wooden dowel used as fork-blade brace for shipping, but since it's been hijacked to the practice of "wooden plug inserted into base of steerer" and cross-drilled for brake through-bolt...
This was indeed a common practice with primarily French manufacturers, but also known to have been done by a few Italians, as well. Didn't ever catch on with Brits, AFAIK. It was not reserved only for "racers" but that's who it was aimed at.
The theory (debunked or not) was that since many forks were seen to fail at the juncture of the steerer base and crown, and since such failures could be swift and devastating to the rider, this practice of fitting a tight hardwood plug into the base of the steerer (and drilling it to be further held by the brake bolt) was adopted.
The desired safety effect was that, upon catastrophically breaking, the plug-with-bolt would hold the pieces together long enough for the rider to stop safely...but in practice it can be argued that there's very little guarantee that this would work as intended: all depends on how and where the break occurred, and how tight that plug holds to the broken steerer.
It might have worked (once or occasionally), but was probably more "insurance" to the rider than practical safety device.
What I can say from personal experience is that a tight wooden plug does and did function as a great moisture trap and rust generator for the few steerers I found them in.
I say remove them and leave them out!