But as it has been said most BFA plugs are made of steel with plating.
I don't know who said that but there is no steel (iron) in beryllium copper or "spring" copper. Maybe the word spring makes people think of steel. Beryllium copper is generally made from copper, beryllium, cobalt and nickel. It has less conductivity of course than pure copper, but still a good choice for electrical contacts requiring spring tension qualities.
I haven't personally used the Furez copper plugs, but the elastomeric insert is one interesting and possible method to provide outward tension on the almost pure copper split tube.
"Plastic" is a very wide and general material classification, but more specifically, elastomeric or viscoelastic polymers such as Sorbothane have been around for ~ 35 years and so far have shown excellent resistance to age hardening or cracking.
Steve