The 8042 had another "fun" function on the 286 too. Because there were a number of poorly written programs that expected the memory to roll over at 0x0FFFFF, in order to maintain compatibility, they needed some way of turning off the A20 line. Now, in order to save money, IBM noticed that there was a spare pin on the 8042 that they used to control whether or not the A20 pin would be held low or not. Now, because this became another bit of needed backward compatibility, a way to assert the A20 pin stuck around until Intel's Haswell line, several decades after A20 gating was needed.

Also, there was another way of getting the processor back into real mode that was even more fun. You'd initiate a triple fault to trigger the reset, because that was faster than asking the 8042 to do it for you. Oh, the good old days.