Generally speaking organisms of today retain the adaptations of their ancestors, at least to a measurable degree. Whatever organism that ate plastics back then

Generally speaking organisms of today retain the adaptations of their ancestors, at least to a measurable degree. Whatever organism that ate plastics back then is unlikely to have gone extinct but rather gone back to whatever it was eating before, while retaining the plastic metabolism. Within the span of hundreds of thousands or millions of years (e.g. when a prior evolved primate would have had their technological period) those adaptations would probably remain intact and their descendants would be munching away on plastic today. If we suppose that the plastic culture existed further back than that, then there might not be organisms with intact metabolisms, but nevertheless there'd be some residual proteins that had an enzymatic decaying effect even if the organism didn't make good use of it. The fact that plastics have shown no deterioration at all for extended periods of time seems to indicate that nothing which survives today in contact with plastic has any lineage back to organisms which once consumed plastic in the past.