I've carefully searched through SE and web and haven't found the exact answer.
This 2018 announcement states that new plant parts are made of plant-based polyethylene. It also implies that plant parts used to be made of polyethylene too, but it's just a hint though. I've also encountered some sources on the web mentioning polypropylene as the material.
Here are the examples of parts which seem to be manufactured of the same sort of plastic:
- 2518 Tree Palm Leaf Large 10 x 5
- 2536x Tree Palm Trunk
- 2542 Paddle
- 30158 Backpack Opening
So now I wonder which material soft parts (plants and similar) are made of prior to the 2018 announcement of switching to plant-based polyethylene.