Could you cite where in 5e it's RAW?
It deals with skills. If you look at the skills section of the PHB, you'll see that knowing something about the inhabitants of the planes(Demons, Devils, etc.) is part of the Arcana skill. Knowing something about animals falls under the Nature skill. And so on. It's pretty clear from the skills section that knowledge of things like creatures is an ability check, not something that the player can simply have his PC know just because he knows or wants his PC to know.
Also, the whole having your PC know things that you know just because you as the player want to know it would also apply to every other skill regarding knowledge. If you the player knows a piece of history, you can do an end around the ability check system and just know it. But that's not how ability checks work. There's no part of that system that says, "The DM will decide the DC or you can just have your PC know it if you want."
The rules for ability checks in the PHB are that the DM only calls for a roll if the outcome is in doubt. So the two examples I gave above where you auto succeed and auto fail fall under the "no doubt" portion. If the outcome is in doubt, the DM can call for a roll. The DMG further clarifies by saying that you should only roll if the outcome has meaning(or something like that).
The section you guys brought up regarding metagaming isn't really talking about the sort of metagaming we use here as "Bringing player knowledge into the game that the PC wouldn't have." Rather it's more along the lines of...
DM: As you crest the top of the mountain you see a valley far below. At the bottom of the valley you see 5 ancient chromatic dragons. One of each color, standing in a circle. They seem to be engaged in a heated discussion. One of them glances up your way, and then returns to the argument. You hear something like, "....internet...," but you can't be sure.
Player: Well, the DM wouldn't put an encounter into the game that we can't handle, so they're probably shapeshifters or illusions or something. Let's go get em!!
That's the sort of metagaming that the DMG is speaking about. It's talking about the game as a game, rather than bringing in knowledge that the PCs wouldn't have.
It's not that the jerk attacks the werewolf with silver without the DM knowing, it's that he "pretends" to discover it after another character fails to damage the wolf. Then it becomes a game of "Wait, I think you already knew that." "No, really, I just thought of it." Etc.
Yes, that's being a jerk (if the social contract of that game expressly forbids it.). But that's my point: both playstyles are vulnerable to jerks. Therefore neither side in this debate can rely on the jerk argument.
I agree that both(and all the other) playstyles are open to jerks. That's why I don't really consider jerks when discussing playstyles. Jerks are a jerk issue, not a playstyle issue.
I meant "necessary" in an objective sense. If a group of players decide that they must always voice act when speaking aloud, that by definition is "necessary" at their table, but that does't make it necessary for the game to function well.
Well, in that sense there's very little in D&D that's necessary. You need players. You need a shared imagined space. And you need some sort of "race" to be able to interact with that imagined space. That's it that I can think of. You don't need monsters. There's no specific race that's necessary. You don't need any specific terrain type or even air for that matter.
Also, maybe it's quick and easy for you because you know FR well, but as I think the OP's story illustrates, the "no player knowledge" approach causes problems if the players know more lore than the DM does, which effectively creates a barrier to entry for DMs. The "player knowledge is fine" approach doesn't have that problem.
I let my players know that I depart from the lore in areas. Some things I don't like, others have changed via play with other groups. If you let the players know that not everything is as they expect or set in stone, they tend to stop doing things like that.
