Rear brakes do very little except for parking and hard braking. To keep the rotors clear of rust, if it has a hand brake, you can pull up on the brake lever, keeping your thumb on the release button. Do this while driving with nobody behind you. You’ll be applying only the rear brakes. They won’t be very effective - but the pads will be brushing rust off the rotors.
In normal usage, there is seldom that amount of braking force applied to the moving rotors, so they are likely to have surface rust and if they are old, some pitting.
If you feel pulsation while doing this, that’s a sign of warped rotors. Since rear brakes do so little, warpage is not very likely. If the vehicle pulls to one side, the other side is not braking well.