Mannahnin
Scion of Murgen (He/Him)
But metagaming CAN be about coming up with good ideas. Again, there are multiple definitions in discussion here.It's not about coming up with good ideas. It's announcing to the table that "This is an X, therefore it's immunities are Y and it has legendary actions A, B and C and it recharges attack D on a 5 or 6". It's "We go down the left corridor and the third room where we find a chest. We'll have to disable the trap first, but there's really cool loot in there. Then we can go to...".
If, as a DM, I think killing trolls with fire with common knowledge (it is in my campaigns) then it is. If I think you have no possible way to know a marilith when you see it, you don't. If your PC has no way of knowing exact position of every creature on the board even though I've left the minis on the table then you don't get to use the knowledge of mini location to cast that fireball so that it only hits enemies.
It's about at my table I want to assume the role of the PC and interact with the world as the PC the best I can. I want my players to make a reasonable effort to do the same because it's the expectation several people at the table have. If you and yours don't care then it's not an issue.
The example with the Intellect Devourer and Protection from Evil is a good one. It's a legitimately ambiguous case. Would the character reasonably know that or not? How do we decide? The DM in the real world situation was unable to decide and kicked the decision back to the player, who was trying to be a good doobie and aware of his own conflict of interest. So how do we choose in a situation like that? Do we just always default to assuming the character is ignorant? Does that actually make the game more fun?
Other posters have pointed out that "metagaming" is not only defined as "using game knowledge the character would not reasonably possess", but "thinking and making decisions about the game itself, outside ways your character could", like making action decisions based on the initiative order. There's no way your character could know what the initiative order is, but unless we do before-initiative action declaration, there's no way for the players to avoid using metagame knowledge in planning their turn & actions.