Brake fade could easily have been left out because they tried it and it wasn't fun or confused players. Accurately modeling the cornering, acceleration and other physical properties of the car's motion are central to what we call physics.
Brakes aren't important to accurately modelling the physical properties of a car? Huh. Are you one of those players that just bounces off walls to get around corners? Because the way I see it the brakes are about as important as the engine, they're both highly involved in how the car speeds up and slows down which is where about half the gameplay is.
As far as tyre deformation goes, apparently it hasn't been that critical to the physics for GT1 through GT6. Is it linked to the physics in GTS? I don't know. If it isn't though, I don't see why it would be for GT7. There's plenty of "minor" physical things that Polyphony doesn't model for whatever reason. It's a simcade, not a full on simulation. As you say, maybe they tried these things and found that they weren't fun or were confusing. I could certainly see that for tyre deformation, it's
much more complex than variable tyre pressure and they don't model that.