It's metagame knowledge if you use it for your character when it is unlikely that they would know it. Which matters to some folks, including me. Particularly me, since as a forever DM, on the rare chances when I actually get to play, I know most every monster, magic item, spell and so on but it's not much fun to play my character as if they share that knowledge.
Thing is, the characters live in that world. They
should know things about it. Using trolls in the Forgotten Realms as an example, the Sword Coast has multiple areas named after trolls (Trollbark Forest, The Trollclaws and Trollclaw Ford, Troll Mountains, and the Evermoors are so named because of the trolls that just won't stay dead), Waterdeep has a section of the city walls named the Trollwall for all the trolls they fought there, and even a public holiday commemorating the Trollwars ("On this day commemorating Waterdeep’s victory in the Second Trollwar, children run through the city acting like trolls, banging on doors and growling, from highsun till dusk."). So I think the idea that people in the Forgotten Realms wouldn't know that you need fire or acid to deal proper damage to a troll is ridiculous. Same thing with the "common" dragons and their breath weapons and associated elements. People
should know that green dragons breathe poison gas. Of course that doesn't mean they should know everything – for example, green dragons used to have mind control abilities that got nixed from 5e because we can't have complex monsters, but that's the sort of thing that might not be common knowledge.
Now, if we're talking esoteric planar monsters, that's a different thing. I think people with some education would know basic things about them like devils = tyrannical fiends who ensnare people with deals and demons = crazy murder monsters who are only in it for the destruction, and how (in 3.5e) you want silver to deal with devils and cold iron for demons. You probably also know that they can teleport and summon allies. But you might not know whether a barbed devil or a bone devil is higher up in the hierarchies of Hell, and what
precisely they can do. And you probably don't have a clue about whatever a hollyphant does.