I have some photos from a low mile car which shows the plastic wiper blade assembly used in 84 on an GT. My 84 SVO had the plastic ones as well but I believe they are stored away.
I recently replaced the wiper blade inserts (E6FZ-17593-A) on my wife's low mile 88 McLaren and it had the metal blade assemblies. Incidentally, the rubber inserts still had the 88 dates on them.
The Motorcraft replacement blade assemblies have been metal for quite some time WP16 I believe. They recently disappeared from production. They are easy to identify as the have a UV exposure decal which turns yellow with age.
Wiper Blade assembly
79-81 D9zz-17528-a,16.5" Tridon
81-87 E3DZ-17528-c, 16" Tridon Plastic
88-? E6FZ-17528-a, 16"
The MPC has listings for the blade inserts. I'm not sure if the plastic and metal ones interchange.