Last time I saw unleaded was in about 1986, when I got rid of the Audi Fox (blast to drive, what a rustbucket!). I've not looked since, but leaded regular vanished shortly after that.
Premium rating (and octane) have nothing to do with lead content, except that tetraethyl lead is a wonder octane booster, used in moderation -- that is, if you ignore the serious toxicity problems! Premium went unleaded in the late 70s, I think, certainly I found that Amoco had unleaded premium (92 octane) in 1980 while I was still driving the Toyota Crown, which used 95 min. I traded it for the Fox when Mobil closed their last station in Evansville, since they were the last one (except Sunoco, and I didn't like their fuel) to sell 95 octane leaded premium.
Unleaded fuel has been around so long now I don't remember when it started, certainly in the early 70's.
The excessive valve wear thing was, I think, blown all out of proportion by Ford and GM, both of whom were still selling engines with valve seats ground directly into the cast iron head, with no insert. They also blew enourmous smoke about emissions controls, only changed their minds when California outlawed internal combustion engines the first time (1965).
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!