Fantasy world might not be earth, but generally on earth you can still see the horizon some 3-5 kilometers away, more than sufficient to handle any dnd spell that actually has a target where this clause matters.This makes me think, anything once you reach a certain distance away would have total cover, if from nothing else than the curvature of the earth.
So are there any spells that "target" things reasonably far away, that under this reasoning would no longer work?
Another thought, aside from the globe wall of force, couldn't magic that wasn't a "physical" thing just go around the wall?
For example, I can see something standing on the other side of the wall, why cant I charm person it? I mean magic doesn't have to follow straight lines in all examples, or things like sending a message to a person on another continent wouldn't work.
In terms of cover, here is the relevant passage:
CoverWalls, trees, creatures, and other obstacles can provide cover during combat, making a target more difficult to harm. A target can benefit from cover only when an attack or other effect originates on the opposite side of the cover.
The rules don't allow for the notion of a "curveball", if the obstacle is directly between you and the target....its total cover.
As far as the sending spell goes: It has an unlimited range and no target. Therefore, the rules regarding targets and cover does not apply, which is why you can send to anyone anywhere.