I would learn how, it's a great skill to have and there aren't a ton of folks that can do it. You can buy body lead or body solder kits from places like Eastwood, the stuff they use now contains less lead than it used to, so, maybe marginally safer. It's not super hard, but it does take practice to be good at it.
The factory used lead for a reason at the quarter/roof and tail panel/quarter seams. Bondo will crack at those locations, there's too much flex. Lead doesn't. The factory could have used bondo in those locations, and instead slung lead. At least on the non-vinyl top cars. It would have been cheaper and easier for them to just bondo all of them, but they didn't.
So is it something still used on old cars today? Yes, if you're trying to do it the way the factory did. Most folks just sling bondo though. Like I said, if you have the opportunity to learn how to do it, learn it. If you don't use it, that's fine. But it's another skill for the tool box. I just did the lead work on the tail panel seams on my Duster. Even my '74 Duster had lead filling in the quarter/tail panel seam. I melted it out when I did the Demon tail panel conversion, and put it back because I learned from my old man, who did it on British car restorations. I'm not great at it, but I was able to get the job done. I left the finish rough in the last picture because I do a little heavier skim coat over the lead work, so when it comes time to be sanding on the car for body/paint work I'm not sanding on lead. You do not want to breathe fine particles of this stuff.
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