Plastic can be toxic...see BPA a building block for certain plastics.
BPA is a known endocrine disruptor, and numerous studies have found that laboratory animals exposed to low levels of it have elevated rates of diabetes, mammary and prostate cancers, decreased sperm count, reproductive problems, early puberty, obesity, and neurological problems.
BPA is a xenoestrogen, not the only one to be found in plastic manufacturing, not the worst one (many BPA alternatives have higher estrogenic activity), and is also not the only type of chemical to look out for; phthalates, for example, fuck with testosterone metabolism, and are often found in plastics, too.
So, yeah, there is definitive proof of it being a problem, and studies have been published in depth for quite some time. You kinda missed the boat on this by about a decade.
the U.S. Food and Drug Administration (FDA) has said that BPA is safe at the very low levels that occur in some foods. This assessment is based on review of hundreds of studies...
https://www.mayoclinic.org/healthy-lifestyle/nutrition-and-h...