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...".So, metagaming has never really come up as an issue in games I've played or run. Reading this thread, though, I can understand but why the Angry GM got so angry about the topic.
I can't really imagine playing a game where players are required to justify whether their character would have come up with the same good idea they did. Sounds excruciating.
To each their own.
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.