We do have certain 'depopulated zones' in the middle of civilised lands. We archaeologists can sometimes identify them as ancient forests, uncultivated, but why those forests were not cut down and used is not entirely clear. Sometimes cultic activities take place in these forests in later times, but whether they were also for this reason 'taboo' earlier or if they represented physical buffer zones between polities (or both) is unclear. We do know that around the 3rd century AD, large parts of the coastal zone are depopulated throughout Northern Europe; my most likely explanation for this is that they were abandoned due to fear of maritime raiding, which does represent a 'dead zone' of sorts, though not necessarily between polities.

Textually, Carlin's description is copied from Caesar's Bello Gallico 4.3.1, where he describes the practice of empty zones amongst the Suebi. However, the establishment of buffer zones is also a conscious policy of the Romans themselves. This paper suggests the Romans copied a Germanic border policy, but I now wonder whether the influence was not the other way around: the Romans projecting their own policy on the barbarians. In my experience as an archaeologist, Germanic societies were more concerned with personal ownership and access rights to grazing lands, and not communal tribal lands. Proto-state-like behaviour like formal borders does not really feel like their thing to me, and our only real evidence comes from Roman authors. The final verdict hinges on how much you are willing to take the classical textual sources at face value.