This is absolutely not true. In the 1950's food was packaged in glass or paper. Polymer science was in its infancy.

Fair point. Still, I think the reference point for plastic ubiquity comes quite a while after the 1950s. Plus, there are many different types of polymers and plasticizers and processed food is exposed to plastic at many points during its production and packaging. I would expect to find very different microplastic concentrations in leftovers from a home-cooked meal stored in a 1950's tupperware container and a modern frozen lasagna (or whatever) microwaved in its own plastic packaging.