Ovinomancer
No flips for you!
To riff on this, I think that if the DM is mature enough, they're not going to craft encounters where making the players pretend to not know things is necessary to make an encounter interesting. In other words, trolls being vulnerable to fire shouldn't be the crux of the encounter. If you build encounters such that this is the important bit, that's on you as DM.No. Unless their background or skills mean they should know.
In my opinion the creators of the Walking Dead had it right. If zombie movies exist then zombies are less scary. We played Curse of Strahd the same way. No characters knew what this blood sucking fiend was, because otherwise their characters are all planning for it.
If the players are mature enough to separate character knowledge from player knowledge I find this method is far more satisfying.
And, as you may guess, I see no point whatsoever in making players pretend they don't know stuff and fumble around for however long the DM thinks is appropriate before they're allowed to try fire on trolls.