Tintin i did some digging & i think this must be referring to the discussion of "shadows of things to come" in Hebrews (and assuming that Paul is the author of this letter).
because of the vocabulary used, and the dichotomy drawn between the substance of Christ and the shadows that are the former things, the thought is that Plato's allegory, possibly through thinkers like Philo, was known & is in a way referenced in Hebrews 8-10.
of course it's inference and speculation based on similarity of thought, because the letter doesn't include a footnote explaining "by the way, you know Plato's allegory of the cave? well what i'm saying is like that.."