I never write out a NPC's backstory aside from at the most few quick sentences or bullet points. I'm certain there are others who think otherwise, that you should write out a full-fledged biography for your NPCs, but that way you can hide more important details if needed as well keep yourself from putting in too much effort towards a character that is potentially relevant for a few scenes/that arc of the roleplay.

Honestly, it kind of seems like you're worried about your players potentially metagaming, which is a fair point. But part of me doesn't feel like hiding segments of player character sheets is the appropriate action when you as the GM can just talk to or even warn the player about the metagaming issue. After all, you've got GM powers; might as well use them. And who knows, maybe the player has a valid reason for their character to know said NPC (though I imagine they'd probably bring this up to you beforehand).