An idea that came to mind recently, especially for conspiracy ttrpg settings: In the source media, many reveals are about shaking up what is "known" about the pasts of main characters. The mother is actually alive! The employer is actually a member of the conspiracy! The ex is actually a shapeshifting alien! The benevolent order is actually bent on world domination!
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When translating that into ttrpgs and especially trad games, you run into "ownership problems" because of the difference in production: A tv series is developed by a writers' room for the consumption of viewers. No writer will ever be surprised by a twist, no viewer will have ownership over a character (and some viewers really ought to remember that at times).
In ttrpgs, the writing duties and even the consumption are somewhat shared between the GM and the players - the exact ratios and portfolios differ between trad games and story games and even within these types, but it's never 100:0.
That creates a problem: GMs and players are not supposed to unilaterally change established facts. A player can't just say "Actually, this room doesn't contain an orc, it contains a treasure chest." and a GM can't say "Actually, you're not a 13th level fighter, you're a 13th level rogue." Such changes usually need consent and negotiations from both parties and they are again more often seen in story games.
So where does that leave the shocking background twist?
You can't really spring it on players unbidden because it's bad form to just change the part of the world they created.
You can't really negotiate it without losing the shock value of the twist. (Which is not to say you can't negotiate changes to established facts - they just won't be shocking twists then.)
My proposal: The unreliable narration.
Have players write their background stories. Then have them put in italics all parts of these stories that they need to be 100% true.
Cifer
Matthias