Most likely, the key has come in contact with somehting that
affects rubber.  It could be something oily.  Think about
what it may be.

One fix is to replace the key, but they are expensive.
Perhaps you can fit a plastic shield over the rubber.  I
doubt you could restore the rubber, and peeling it off and
replacing it might cause damage to electronics under the rubber.

If it is they key material I'm aware of, I have yet to see it degrade.
It gets scuffed and nicked over time but doesn't seem to decompose at
all.  As the other poster indicated, it probably came into contact with
some solvent, gasoline, paint thinner, etc.  I'm not aware of a way to
reveres the decomposition if that is what is happening.  Have a new one
cut.

Do you use a lot of hand lotion?  My wife does and it has caused the
steering wheel wrap to crack and the center console where her elbow rests
the faux leather cover has disintegrated.  It did this on another car (a
Cadillac) we had.  I don't know if this would cause problems with the key, I
haven't seen hers lately.

Allen