ammonia is a no-no for plastic.
On what issue of Mikey mouse magazine you read it ?
Ammonia does nothing to most plastics, other than a bit of whitening to the older one.
Does nothing also to copper.
That said is still a dumb idea to dump a whole keyboard in the ammonia or in a solution ot it because it attack aluminum which often is used for the matrix, and for electrolytic capacitors) and can attack some type of glues used on PCBs and keyboard matrix.
Is not a good idea also to clean the monitor screen, (no matter if LCD or CRT), because some of the antireflective coatings can be altered or completely wiped away