What material are older soft (plant) parts made of?
According to Brickset, these parts have always been made of polyethylene, the only difference is that some of the material is plant-based since the Plants from Plants initiative.

Can you tell the difference?
No! They look, feel, smell and work just like 'regular' botanical elements. This is because, chemically, they are the same. You can safely mix them with your other plant pieces safe in the knowledge that there is no difference.