It was in the rule book, but they tightened up the wording presumably because someone had found a clever way to interpret them. It's gone from:
The brake system must be designed so that within each circuit, the forces applied to the
brake pads are the same magnitude and act as opposing pairs on a given brake disc.
to:
The brake system must be designed so that within each circuit, the forces applied to the
brake pads are the same magnitude and act as opposing pairs on a given brake disc. Any
system or mechanism which can produce systematically or intentionally, asymmetric braking
torques for a given axle is forbidden.
I guess the inertial systems they're talking about aren't covered by the first if you squint funny. And squinting funny at regulations is what F1 is all about