Whenever, I want to restore rubber or vinyl (something which your keyboards' cable jackets falls into the category of) I use Meguiars #40. I've even used it to restore the rubber isolation bushings inside of CD-ROM's. After you've cleaned your cable to the extent you can, I would lay it out and saturate it with the cleaner/conditioner, and let it stay like that so it will help recondition the cable jacket.

The ultimate way to do this would be to lay it out over some plastic-wrap or half of a plastic trash bag, then saturate the cable, then cover it with more plastic-wrap or the other half of the plastic bag (plastic-wrap is best because you can see through it and make sure the entire cable is saturated), then press the plastic against the entire surface of the cable, and leave it like that, perhaps overnight.

Just make sure that anything you cleaned/treated with the #40 gets wiped down very well with a dry towel because otherwise it can leave a slick feeling to everything. (but it's not silicone based, which is good!)

It's not for bringing the white/beige color back, but I love Meguiar's Supreme Shine (G4016). Why? Because it smells like coconut cookie! 😁 And it polishes vinyl/rubber pretty well (yes, it also gives that slick look which may not always be desired).

But for re-whitening, severely yellowed plastic surfaces, I think you're better off with stuff based on hydrogen peroxide, like Retrobrite. Magic eraser was mentioned around here recently but it's abrasive so be careful with that.

http://www.vintagecomputing.com/index.php/archives/189

Material Compatibility with Hydrogen Peroxide (H2O2):
http://www.ozoneservices.com/articles/004.htm