Anything not made of stone will ‘disappear’ within a few thousand years, unless specific conditions are met (like being kept within an air-proof and dry vault, etc), and even then, over a long time, these will disappear, too, maybe aided by tectonic processes exposing a previously contained artefact.
There have been sporadic reports of artefacts found in coal seems (which allegedly are millions of years old),
like this one. Of course there is always a possibility that a) it was all a clever fraud for fame/ publicity or whatnot, or b) that coal seams are a lot younger than mainstream science thinks.
And I am not sure that plastic will not decompose eventually, too, given that it is organic and
certain bacterias seem to like plastic. On top of that it may be possible that Atlantis never really had a need for plastic, as plastic really is mostly a (cheap and readily available) substitute for more expensive materials (their tech was likely vastly different from our).
I think the whole question involves a lot of ‘moving parts’, so to make a fair call from our perspective will likely be very difficult, because mainstream science really doesn’t want to go down that particular rabbit hole and anything that questions the current paradigm will be shoved under the rug.
