I think it was Tumbleweed, by Murphy, which was based on Glyphosphate (same active ingredient as Roundup) - there was a sort of deodorant-style push up stick, and they did some sort of foam as well.

Not sure why it is no longer available if it was Glyphosphate. The "child proof caps" have got very much more secure over the years, and maybe they couldn't do that for a paint-on type product? Or (cynic mode) it used less weedkiller and made less money

I think 2,4-D is still available and isn't it the stuff that is put in selective weed killers for grass? (and therefore probably not quite the right thing for a kill-all product!)

I think the main scare with 2,4D (and its cousin 2,4,5-T) was that the manufacturing process stuffed it with Dioxins, which were bad news , and probably the thing that caused Agent Orange to have been a Bad Idea. Anyway, they can now make it without Dioxins

Still doesn't make it a friendly chemical though!