I’ve done it before and I’m still, in some ways, doing it now.

Either it never pops up in the IC before the RP dies, or people’s predilections for internal monologue ruin the surprise before it’s ICly revealed. It’s a great thing to play with either way though, as the backstory is otherwise a fairly pointless thing to have in a CS, perhaps only useful for ensuring that people read your five pages of lore and setting.

And in the once-in-a-blue-moon scenario where you get a character interaction focused RP that is active and long running, the secret backstories really get to play out in fun ways.

So if you think about it, you honestly have nothing to lose as a GM if you just cut the Backstory portion out of your CS templates completely and then tell the players to toss only the spicy history deets at you.