Not sure where "microplastics" came from, but stopper particles are definately a thing. I suspect most people think some magic system filters those out of your system, when they're far too large for the kidneys to deal with, and there's little your immune system can do about it but try and coat it with cells to isolate it from your body.
And we're not talking about some "natural rubber". The cheap Chinese stoppers are bromobutyl, like car tires, but with more toxic shit to make them easy to manufacture into shape cheaply. Not to worry though, the material is formulated to maintain integrity for its entire 5 year shelf life. before disintegrating into mush and releasing their chemistry set payload wherever they happen to be. Ever touch something that used to be black "rubber" and is all sticky now for some reason? That's bromobutyl.
Luckily it's just a little chunk once in a while, and since injections are rare the risk of accumulating into an amount that could affect you is pretty low. If we were talking a hundred+ medium to large gauge (27g and bigger) injections a year it might represent a risk...but who gets that many shots?