If you tell people their piano has plastic parts, most don’t think of that as a good thing. When plastics first became mainstream in manufacturing in the 20th century, they were used in all kinds of ways they should not have been and have a bad track record for durability. Early plastics became brittle very quickly and had poor shelf life.
However, in recent years a number of piano manufacturers have developed advanced polymers that are used for specific action components and are actually more durable than wood. They are also far less susceptible to seasonal temperature and humidity swings, helping the piano’s regulation remain stable longer.
This picture is of a “plastic” repetition jack in a Yamaha upright that’s just a couple years old. For decades this part would have been made out of wood. It is a quality part though and no cause for concern! You’ll notice other parts are still made of wood and metal; it all depends on the function of the part.