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Does anybody know when the first non-ivory naturals were made? What kinds of plastics, when were they introduced, how to recognize them?

Prior to ivory, various hardwoods were used, but they would scour fairly quickly. I believe the first alternative to ivory was celluloid, and while I don't know when it was first introduced, I see pianos with them that date from at least early 20th century. Plastics came later. I'm sorry I cannot provide you with dates.

Celluloid typically has a slight grain to it. Very straight and even, and because of the age and texture is sometimes confused with ivory. Ivory grain is closer to wood grain...less straight and even.

Sometimes it is easy to identify them in how they age/fail. Ivory darkens and discolors, but can be restored by cleaning + UV treatments, or sanding and buffing. Early plastic keytops are not colorfast and typically have yellowed significantly by now. I've never tried to save or repair celluloid key tops. Most early plastics I see are either very thick and have soft rounded edges, or they are thin and usually warped and turned up around the edges. Sometimes the thicker ones can be improved with sanding & buffing. All 3 are prone to chips at the edges, but there is no grain to the plastics, so small cracks are less common. I don't recall seeing cracks in celluloid key tops.

Hopefully others can add more specifics.

In some new pianos the keys are a rather slippery plastic more so than others .There are also much better ones which do not seem
so plasticy and are more enjoyable when playing .
There are also some made of ivorite. I do not know what this is .I presume an imitation or synthetic ivory ?

Plastics have greatly improved, and there are various material options for a more textured feel on the key tops. Composites, or "fancy plastics" if you like, are available from several sources. Yamaha & Kawai have trademarked their formula for their own use...the formula changes, but the trademark stays. Other materials include bone, mineral, and even mammoth ivory, though I don't think that avenue has gained any traction for a variety of reasons.

Sharp materials include ebony, plastic, composite wood (essentially wood fiber and glue) and other hardwoods dyed to look like ebony. Even ebony is dyed for a more even finish. There are also layered wood sharps for added visual interest.

What is interesting is that after so many years without ivory, the public reaction to the upgraded surfaces is actually mixed. I like a nice ebony sharp and a very lightly textured natural. Nicely preserved and refreshed ivory is nice under the fingers, but most of it isn't worth the trouble or is unfortunately stuck to a piano that isn't worth the trouble.

Sorry for getting a little off topic. I think it is important to note that ivory was chosen because there wasn't a man made alternative yet to do the job, not because it was exotic or desirable. Phenolic resins have come a long way, but arrived late and developed gradually during a "key" period in the history of piano making.

The general evolution of natural keytop materials went something like this:
wood
ivory
celluloid, (nitrocellulose)
ABS styrene
acrylic
specially modified proprietary plastics, (mineralized plastics I believe)

For sharps:
various woods
phenolic plastic
ABS styrene
Composite wood fiber

Celluloid was used for a long time. I have seen it on pianos from the late 19th Century, although I do not know the exact dates. It became almost universal for key fronts very early, as ivory key fronts had lots of problems.

It was not until the mid 1950s that other plastics started showing up. By the late 1950s, the US manufacturers had switched away from ivory. Some of them still used celluloid.

Celluloid was used for motion picture film from the very beginning of the movies until the early 1950's. In that application, it had two real issues: It was highly flammable, and it would after a few decades decompose into a sticky smelly mess. Has anyone ever seen celluloid key tops that decomposed into jelly and smelled like old gym socks combined with moth balls? (OT, how many of us are old enough to remember moth balls? ;-) )

What about "Bakelite"? It is a phenolic plastic dating to about 1907, used in a huge variety of stuff for many years following. I have a few pieces of colored costume jewelry made from it, and some dressing table pieces which look like yellowed ivory. I know it was used in old toaster levers. The stuff lasts intact for an amazing length of time. For a quick analysis:

http://discovery.kcpc.usyd.edu.au/9.5.1/9.5.1_ivory.html

Bakelite was used in musical instruments other than pianos, too, and according to this website, as a variety of piano parts:

coppellpianoshop.wordpress.com/2012/02/22/pianos-and-polyoxybenzylmethylenglycolanhydride/

Weren't the cases of the old "black box" Franz metronomes Bakelite?

I listed phenolic as used for sharps.

It is used for the fallboard hinge on many Mason & Hamlin grands, Music desk catches on "newer" (starting post WW2 I think) Steinway model S,M,L, and O. And appears in other piano applications sometimes.