[dTal]

You're correct of course. It's wildly infeasible, prone to rapid detection, and there's no plausible profit to sneaking subtle backdoors into random KiCAD projects. Now let's brainstorm possible vectors anyway...

You could manipulate PCB traces in such a way as to leak data over RF? No topological change to the circuit and very hard to pin down.

[polishdude20]

Any data leaked would just leak more than it already does. If your data is encrypted, it would just be encrypted but louder.