Mfgrs moved away from molten lead for health and safety reasons.
Making plastic filler look right for paint took a lot more time than the lead did.
That's why they pushed vinyl tops. It hides the flaws.
My old 68 charger had lead filler under its original vinyl top which i removed, and then repainted the car leaving the vinyl top and trim off. My old 69 charger R/T when i pulled the vinyl to repaint and revinyl had cracks at the seams, and was plastic filler. So i would assume since both cars had factory vinyl tops the change to plastic filler in the seams probably started in 1969.
