100%. I've been in homebrew settings where they don't even exist. They also didn't exist on Athas. They might not in fact be common in any particular game.
So, in a setting where they never exist, why would it matter? They don't exist, so you wouldn't fight them.
Also, if a guy comes up to you, why would you suddenly question "Is he resistant to fire? He looks like a normal dude, but I can't possibly know if he is resistant to fire?"
Way to ignore the other three races you lumped in with humans and elves. Very telling that.
As for common. They too might not even be in any given game. Or might be exceptionally rare and not a PC race.
Oh sorry, I forgot that I was dealing with you.
Humans -> The people playing the game and a playable race in the PHB
Elves -> A playable race in the PHB
Goblins -> A playable race, also an insanely common creature throughout all of folklore, myth and fantasy. In basically none of those depictions are they resistant to energy damage.
Wolves -> A real-life creature, who is not particularly resistant or immune to things like electricity or fire.
Owlbear -> A creature defined by being the blend of two real life animals, neither of which is particularly known to be fire resistant or able to avoid being electrocuted.
Now, sure, MAYBE these things are either the single most common fantasy races in the known world, are real life things, or are combos of real life things may not be common in any given game. But, again, you act like this is going to be some grand cheating strategy. But the thing is, people and animals exist in a DnD world, and normal people and normal animals have no resistances, immunities or vulnerabilities. And they would know that.
And this, again, doesn't address that I'm now supposedly metagaming by assuming a baseline of nothing.
Magma is a very thick fluid. It would seem to me like a mace might just scoop a large swath of the body out and flick it away doing extra damage.
Really? So I'm supposed to assume that because magma is thick my mace can scoop it and deal normal damage? And if I don't assume that, I'm meta-gaming? So, this isn't about "what do my character's know and how do they use that knowledge?" it is "What does Maxperson think is reasonable and how I should act?"
This is a fundamental problem with calling people out for "meta-gaming" is that at a certain point, you are just calling them out for using different logic than you. And you don't get to police other people's thoughts and conclusions.
So now you're limiting it to undead?
Are Animated Suits of armor undead? Also, it is very telling that instead of addressing the logic, you decide to attack what you perceive to be a moved goalpost.
No. Nor is every case of assuming something that is true a case of metagaming. As I said, if it makes sense for the PC to make that assumption, that's fine. If you have to look for a weak justification, you've walked into metagaming territory.
And none of the justifications have been weak. So why are we having this long multi-stage "I can prove my logic is better than your logic" discussion?
Once per type of monster isn't niche. Sure, if you could only use it once in your PCs life on one monster, that would be niche. DMs use many, many different kinds of monsters, though, so even if you don't personally like it, it will be very useful.
Okay, tell me this. Would Hold Monster be useful if it could only be cast on a single type of monster once? You get to hold one demon, ever. One Devil, ever. One Elemental, ever.
Would this be a good ability? Paralyzation is very powerful after all, and DMs might use many different types of monsters.
You mean that giant wizard?
The one that cast no spell and still called lightning to his hand to throw? Whose eyes blaze with lightning and whose realm is shaped by the storms he calls?
Yeah, him. Probably won't be effective to shoot him with a lightning bolt. Just a hunch from all the "I control the weather and elemental power", it also helps that they faced a Frost Giant before, so the idea that the giant's can be elementally themed is kind of a given at this point.
So creatures that are from three wildly different planes are like creatures who are all from the same plane?
You mean three neighboring planes whose only difference is alignment? Yeah. Kind of comes from that whole "is a fiend" thing.
Assuming incorrectly can bite you in the rear. I wasn't limiting that statement to demons and silver weapons.
Such as....?
Come on, what is the worst that can happen if a player assumes that an enemy is resistant to fire or lightning who isn't actually resistant? Where is the rear biting?
It varies. It could be as simple as something in your background about growing up near the troll moors or an uncle who was a troll hunter. It's very probable that he would know about troll regeneration and what to do about it.
Which has zero things to do with resistance, vulnerability or immunty. Don't make this about troll regeneration, this has nothing to do with troll regeneration.
Why is "lives in ice and fires ice from its body" not a good reason to assume it won't be effective to hit it with cold?
I'm the DM. I can't cheat. If I want to allow the troll's vulnerability to fire and acid to be revealed by a power that informs a PC what a monster's vulnerabilities are, I can.
Sorry, in this example you aren't the DM, you are a bard player. You can't assume an ability works a certain way for a playtest just because you plan on rewriting the rules. That just leads to a naughty word playtest.
AC, HP and average damage are not in-fiction things for a PC to learn. Those are OOC things. Special abilities, vulnerabilities, resistances and immunities are.
They are absolutely in-fiction things, and also... who cares? Why does my game mechanic need to be limited to only learning about Maxperson's approved list of game mechanics instead of learning about any game mechanics we want?