Having high detail vertical slices of worlds is not an imperative in immersive sim games, but practicality often dictates it.
There's a lot of arguing about what constitutes an immersive sim (i.e. NPC interactivity is not one), but the two common concise definitions are "if you can open a door in several inventive ways (including bypassing it) and more generally, if you have so many complex systems in a game that they interact in unexpected ways. Purists also require first-person perspective, but others would also count Hitman or EVE Online.
There are also "sandboxy" games like the open world Zelda, with a chemistry/physics system and multiple-solution puzzles, but wouldn't quite fit the prototypical immersive sim, as there is
still fairly limited NPC interaction. Maybe those count just as systemic games in the wider sense.
There's a lot of arguing about what constitutes an immersive sim (i.e. NPC interactivity is not one), but the two common concise definitions are "if you can open a door in several inventive ways (including bypassing it) and more generally, if you have so many complex systems in a game that they interact in unexpected ways. Purists also require first-person perspective, but others would also count Hitman or EVE Online.