The game advices to set the DC by the difficulty of the task, not by it's relevance to the 'plot'. Granted, it doesn't go into much detail in this and it should. But I generally feel DMG is rather lacking in useful advice department.
OK, that's an objection, but it isn't No True Scottsman... Honestly, the problem here is that the DMG says "set the DC by the difficulty of the task", but what is "difficulty of the task?" There IS NO ACTUAL UNDERLYING WORLD, so there is no cause-and-effect structure to create said difficulty! The difficulty is thus simply a number, which itself now has to be created out of SOMETHING. Since the GM is in complete control of what the fiction is within the world, he's in complete control of the value of this difficulty that sets the DC. So the end result is exactly the same, the GM decides, based on some unstated criteria and in some unstated way, an adjective. He then calls this adjective 'difficulty' and sets a DC according to the 'rule' in the DMG. You point to the rule, without analyzing the entire logical structure, and claim it is a 'simulation', but it isn't, its just a number attached to an adjective that was set through some entirely opaque process that has nothing in particular to do with anything that might be getting 'simulated'. See what I mean?
OK. So what would this sort of mechanic actually look like? What games use such mechanics?
OK, the Space combat system from Classic Traveller has elements of being a simulation. There is some actual attempt to portray weapons and tactics which are analogous to things that reasonably can be extrapolated from known physics and engineering. It is still a game (maybe not a very good one actually, and partly because it is fairly realistic) but there is at least some sort of pretty consistent stuff in there, like the further away you are from the enemy's laser turret, the less probability there is of a hit (IE the angular resolution of his aiming system is a fixed number, but the arc subtended by your vessel at a greater distance is smaller, and evasive maneuvers are proportionately more effective, etc.). It is at least an ATTEMPT to inject some kind of correspondence between realistic things that might actually exist and happen and things that represent them within the game and how they exist and happen.
Now, I agree that, in some trivial sense, falling damage in D&D 'simulates' falling, sure. However I think it is much more driven by the 'falling trope' of dramatic fiction than by an attempt to actually match with physics. Meanwhile, the Traveller space combat system actually AVOIDS the normal tropes of things like space opera where everything is blasters and N-Rays and tractor beams or whatever (queue Star Trek, you can immediately see the difference). I mean, sure you can find a few rare SF books who's space combat might be similar to Traveller's but CLEARLY the thought process started from "what would space combat actually be like?" and the rules grew out of that. Honestly, I think the author probably wasn't very knowledgeable about subjects such as sensors, control system, C3I, etc. and chances are his 'simulation' is pretty bad! Still, its an attempt.
So, as a final point. I can't say whether or not every single thing in every FRPG is or is not a simulation. Nobody can do that, its not a clear line. I really don't think the idea of simulation does much good work in these games though. I think it is MUCH more productive and useful to think about how these things work in terms of producing story logic and necessary structure in terms of orienting participants and giving them indications of what to expect and what inputs will produce what sorts of outcomes. When we build rules structures like the D&D combat system, they map onto story logic, and they orient the players, and the conventions and structure of them allows us to know what is likely to happen and what the different 'moves' we can make will do. When an RPG hews fairly closely to a logical structure that has some correspondences with things we understand in the real world "swords hurt you" then we can also go beyond the board-game-like structure of typical games and into an open-ended mode where the players can suggest moves beyond what the system/fiction already cover. The GM is then expected to respond in a way that is expected and justified, either by an appeal to 'fairness', 'playability', 'genre logic', or a very loose kind of analogizing with real world situations. I think that analogizing is so loose, and shallow, that calling it 'simulation' just doesn't do useful work. As I pointed out with Traveller space combat though, you can find SOMETHING that is a bit 'simulative in nature' here and there, and that means it is hard to be really absolutist.
