/commence to tar pit myself in a pedantic and unnecessary internet argument about how European history informs the verisimilitude of a fictional place
It would be more interesting if you provided maps from those time periods to see how they depicted their own borders as opposed to the modern estimates.
The point being, it's not possible, since prior to 1507 accurate maps (and hence clearly defined boarders) did not exist.
You guys are right about the maps;
Medieval maps generally don't display clear borders. But that's not
@tetrasodium 's main point.
That point:
[Medieval and early modern European polities] sure as heck knew what lands they taxed patrolled & clamed as their own along with the rough area where their foes allies & trading partners did the same to the point where they could point at them on a map.
And the point is fundamentally correct.
People in other times were no less intelligent than we are (
Flynn Effect notwithstanding) and they were pretty good at knowing the things that they needed to know to live. A villager in rural France might never have seen a map, but she would know who the local lord was, who the ruler of the nearest city was, and who the king was--because these would have been important things for her to know. Similarly, the king--even if his maps were rubbish--would know which cities, towns, and productive regions were under his control, what geographical features demarcated the borders of those places, whether control of them was contested (and by whom), and what the neighboring polities were--again, because it would have been important for him to know those things for dynastic security, tax collection, and/or levying soldiers.
There are, in principle, borders that could be drawn on a Sword Coast map, depicting which areas each polity de jure or de facto controls and understands itself to control.
The FR - especially the Sword Coast - does not have fully-developed modern States. But the Realms does have an excessive number of monsters that would eat most attempts at Border Patrol patrols. You don't see lines on the map because "how far does City Lord's writ reach?" is fluid.
"How far does City Lord's writ reach?" is also fluid in 2020 Afghanistan, Syria, Morocco, Yemen, DRC, Sudan, Myanmar, Ukraine, etc. We are still able to draw borders that demarcate the nominal territorial control of those countries.
But at bottom, the Sword Coast isn't supposed to be a plausible quasi-historical place. It's FR's Middle Earth pastiche, just as Cormyr is the Medieval France pastiche, Mulhorand is the ancient Egypt pastiche, and so on.
/end pedantry
I'd personally like to see an adventure set in and around the Moonsea or the Unapproachable East.