Like the example i gave in the post you quoted, if you think you have a valid reason why your character wouldn’t be scared by X or Y I’d be genuinely open to hearing your argument in that situation, but to me, you don’t get to just claim fearlessness by yourself.
Absolutely. I'm 100% in favor of us having a conversation that establishes why your character might genuinely be immune--but it has to be a
genuine conversation. That means both sides have to be open to working with a solution, but both sides also should give a full-throated defense of their position. It also means that neither party can be engaging in bad faith, both have to respect each other and not ask for things that are beyond the pale or offensive.
So, just to quickly invent something, let's say I've got a player, Luke, who plays Ekool Ja, a lizardfolk mercenary and tracker from a faraway land, is faced with a creature that is widely feared for its loathsome, horrendous breath. A breath so foul it can debilitate--blind, enervate, hobble--for a minute or more from a single momentary exposure. But Luke says Ekool Ja is from a faraway continent, where no such creatures exist. He has never seen one before, and because they're not exactly commonplace, nobody has ever explained them to him. Luke asks if Ekool Ja can get an automatic success, not because he isn't capable of fear, but because his ignorance blinds him to the truth of things until he's seen them with his own eyes.
If I heard that, you bet your BRITCHES that I'm taking that offer and running with it. That player has just handed me a golden opportunity. I can now intentionally feed them misleading information anytime there's anything social or folkloric or the like, because it's an established part of Ekool's story that he's a stranger in a strange land. Now, I would use this sparingly and deftly, not to prevent the players from succeeding, but to complicate situations. Even the barest hint of suspicion of overuse would be going too far. But as a bargain with the player? As a "anything you aren't established as knowing,
you don't" offer, couched in how it can help them? Oh hell yes, I'm doing that without hesitation.
That's the kind of thing that can drive whole
campaigns if you let it.