Only the early models came with metals ones. I don't know the date code cut-off, but yep, the all have polymer now. The oldest with plastic that I have seen so far is AH.The HK416 has a polymer ejection port cover.....at least ours do.
It is said that HK found that the polymer ones made for a better seal since they didn't bend. I suspect a big customer had some metal port covers that bent for whatever reason so HK made the change. I will say that the plastic ones are WAY easier to remove than the metal ones, not that you'd have to regulary service that part.