Essentially all by hand. I'd tried to do it a few times before with different quality scans, and while I could see the original intent on all of them, auto-tracing never held up across the whole poster.
I ended up taking a few days zoning out and re-drawing it as a white/black raster image over a highly scaled up scan, then auto-traced that in Illustrator and coloured it there. Text went in as some variant of Century Gothic with a few changes to match the original.
No one seems to have tried to leverage deep learning yet; either because they haven't thought of doing so, or it just wouldn't be worthwhile. image to SVG's are an inherently deterministic task, with not much room for the noisy error of most deep learning models like stable diffusion and such. I think algorithmic approaches are just better in this case - deep learning isn't always superior.
5 years ago... someone tried abusing a text generating RNN to produce broken SVG, and then tidy it up. Genuinely curious as to how this might work with today's transformers.
Anyway, are there any ML-powered vectorizers out there?