With a linear feedback shift register, the input is the XOR of some stage outputs. (XOR is linear when considered mathematically since it's basically bitwise addition.) If you use other logic functions, then it's nonlinear. The 76477 uses a combination of XOR and NAND, making it nonlinear. The disadvantage of a linear feedback shift register is it can get stuck at all zeros. But the 76477's shift register gets set by the inhibit line, so it couldn't get stuck. So I really don't know why they used a nonlinear circuit. As far as I can tell, it just makes the performance worse.