MagSafe Power Cable Going Yellow
Depending on how old the charger is, it is possible the agent causing the discoloration is leaching through from what the casing covers instead of being caused by an external source. In this case, bleaching and cleaning will affect only the surface of the cable.

I used to have several cables like this, on old barrel-shaped (non-MagSafe) chargers, but when the laptops were recycled, so were the chargers, so I don't have anything to dissect to prove my assertion.

The power charger can also get quite hot when charging, and if the charging cable is partially or wholly wound on the charger body's cable-storage fingers, they may become very hot as well, causing some of the discoloration you see. I always fully unwind the connection cable before charging.