Prescription canisters (the tamper resistant/child proof ones). You could do without it but you now subject children to accidental ingestion.

Single meal frozen food containers. You need a seal. Some impregnated cardboard can work, but not as well and is full of chemicals to overcome its disadvantages.

Disposable water bottles. You can use metal vessels, they cost a bit more, but doable.

Packaged frozen meats (they keep longer). Not sure there is a good alternative to plastics other than having people buy meats semi-daily from the butcher.

Some plastics need to be heat resistant, others solvent resistant, others flexible, others stretchy, etc. Different formulations give you what you need at a price point.

Yes we could go back to the 1920s but then life would have to be adjusted to the 1920s. You’d need someone to remain “domestic” at home, etc. as more time is needed to carry out house work without the convenience of modern materials.