If I recall correctly, The original de’oraysah was either on all metals (R’ Dovid Feinstein) or on 6 metals (Rabbi Belsky). Glass is derabannan, since it is melted and forged like metal. Platic was never included in the gezeirah, just like potatoes were never included in the gezeirah of kitniyus. Rabbi Belsky (according to my brother) holds even aluminum doesn’t require tevila, since it’s a metal not included in the passuk, and no gezeirah derabbanan on it.
When I forwarded my brother a text I got that many rebbes want the plane from flight 1549 because it was toiveled, he told me that Rabbi belsky holds aluminum does not require tevila. That opened up other points, like air bubbles inside the plane and the fact that the plane bottom got shredded, so no longer a beis kibbul and no longer a kli.
Bottom line- plastic is definitely not a metal and not glass. It’s a logical extension of a derabananan, but we do not do these things today, unless specifically ordered to by an acharon. (v Yoreh deiah 87 and the machlokes Shach and Pri chadash if we extend the gezeirah of putting fish scales in fish blood to putting almonds next to almond milk). Toiveling plastic is either am haaratzus or a chumra yeseira, albeit a logical one.