I think in this conversation it’s important to remember the emotional weight/impact the flashback might carry, since we’re talking design here, not just play for blades.
In Blades it lets the scoundrel be clever, and elides the need for the players to actually be that clever, or foresighted, but still -feel- clever. It’s terribly important, I think, as an element to let the players feel awesome, which is essential to the fun of the game.
Similarly in a fantasy game, we might want to use flashbacks, but just for a different emotional hit, or it might vary more with character type, if we’re leaning on Tolkein. (It’s an easy frame of reference for me to draw examples, I’m certainly not saying all fantasy stuff is Tolkein, though a lot is.)
So say, Aragorn has a flashback to learning some elf stuff, or to being an edgy woodsman loner type for decades, and therefore has some knowledge or relevant info. This all reinforces Aragorn’s identity, and for the player it helps the character feel authentic, even though in a practical sense, like, c’mon bud, you just pulled that ancient elven lore out of your ass.
Gandalf, similarly, could flash back to hours spent in study in the library of Gondor, or to a confrontation with a dangerous wizard.
As for the hobbits, they might flash back to their quaint hobbit homes, the smell of the river, and all those lovely pastoral elements that give them hope in the face of their impossible task. Again it reinforces character, and is equally powerful, naratively, as the edgy elf lore crap that Aragorn pulls.
It might not be as essential a thing to the genre, but I think there’s certainly room for it. I would just find a way to structure and shape it to suit the needs of your game, and the kinds of fiction you are trying to generate.
Planning is annoying in TTRPGs regardless of genre, in my opinion, because it forces the players to know everything their characters could know, wastes tonnes of time at the table, and is rarely exciting as an activity in itself. Just watch anyone trying to kit out their DnD character with like,s teel mirriors and caltrops and a lantern and…wait how much was the lantern, and oh I need flasks of oil, which are… and I have 6 silver left?
Spare me.