Who really thinks about this? Is it true, (in which case elephants would have to be managed better than they are now)or is it an ivory substitute? And what is the point in a piano key made in wood, overlaid (glued) with ivory? Production wise, not a lot.
Plastic keys are used on digitals mostly. They`re not clever, but would be OK if they had a knobbly surface or something for a bit more grip. And there`s no reason on earth why they shouldn`t be used in acoustics too, imnsho. . . .
There is no good reason for insisting on ivory as a key covering material on piano keys. The simple plastic keycoverings that have been used for some decades on piano keys are quite good. If one doesn't like the feel of the acrylic plastic keycovers there are alternatives. The so-called "mineral plastic" being one. As well, both Yamaha and Kawai offer keycoverings that simulate the look and feel of ivory quite nicely.
There are many reasons why all-plastic keys are not used in pianos. At least 88 of them for each model of piano any particular pianomaker builds.
Digitals (and organs) can easily use all-plastic keys because their keys are very simple and repetitive shapes. All of the Fs are the same just like all of the Cs are the same (except for the one at the end). So all that is needed is one F mold and that key will work for any F in the instrument. Want to change models? No problem, the mold for the F will still work.
Not so with piano keys. Each key in the keyboard is different because the flare of each key is different. And each keyset for each different model is different. A separate mold would have to be made for each key of each keyset. For a company like Young Chang that would mean making at least 14 different sets of molds with each set consisting of 88 different molds.
A secondary problem is with the material itself. Keys for electronic keyboard have very little stress applied to them. Piano keys, because of their length and their flare, are much more highly stressed. They have to be reasonably stiff and resistant to bending. There are, of course, plastics or composites that are at least as strong as the woods being used but these are not cheap materials.
So, yes, plastics or composites could be used to make piano keys of reasonable quality but the cost would be extremely high.