Natural rubber is also plastic. We tend not to call naturally occurring polymers "plastics", but natural rubber is usually vulcanized, so it is not that natural.

Natural rubber from tires is a significant source of microplastics (along with the other polymers the tire is made of)

I'm pretty sure tires are mostly synthetic at this point. I wonder if the natural rubber has a different safety profile than the synthetic, if it is more biodegradable, etc. Even traditional chewing gum fits the definition of plastic, but somehow it feels like it would be safer to use than modern synthetic plastic chewing gum. No idea if that's true

I kind of feel there'd be a difference between vulcanised tire rubber and my drink stopper, but I guess there might not be. Also, tires grating on road is probably a bit different a wear profile. Clearly someone needs to test microplastic shredding in container rubber.

Oh well, at least they have some in that design with cork tipped plugs. It's a bit harder to find though.