Lanefan
Victoria Rules
With this, I agree.Well, let's see.
I explained a thing which is lost by having the GM sew up every single detail of the world before the players ever arrive, an aspect of player agency.
I specified that there really is a value--flexibility, discoverability, authenticity to the IRL cultural experience of pre-modern civilizations--to having a world where, within a certain boundary things are well-defined, probably quite a large boundary, but beyond that boundary, it becomes "HIC SVNT DRACONES", terra incognita. Not only does this add more similarity to what was in fact true (of medieval culture, naturally) in our real world, it ensures that the GM has an important tool for addressing problems that might come up: the freedom to build (not just randomly conjure up, but actually invest effort to create) new elements of the world.
This is something genuinely valuable, even by the lens of verisimilitude, in addition to benefits on perpendicular axes of useful GMing tools and other things.
But I'm not sure of its relevance to player agency.
If all you're suggesting is that the GM keep parts of the map blank for later infill as ideas occur and the campaign expands, that's great advice. All for it!None of this has anything to do with forcing GMs to do anything. Instead, it is about keeping options open.
If, however, there's a secondary motive of having those blank areas be how-where you-as-player can try to introduce elements not otherwise present in the campaign (e.g. "There's no Tieflings in this part of the world, but there could be in that blank area; so why can't I play one who has made the journey from there to here?") then I stop listening real fast.