That... that was mind-blowing.

This discovery is 13 years old. Although it has clear limitations when it comes to humans, I'm surprised I don't see this kind of resize feature available in more software. Does Photoshop have this hidden in a drawer somewhere? Does imagemagick support this?

You will probably see it appear everywhere in 10 years when the patent expires.

https://patents.google.com/patent/US8213745B2/en

What prevents someone outside the US from using this?

I seriously cannot decide if your post is satire. Directly from the wikipedia article:

http://www.photoshopsupport.com/photoshop-cs4/what-is-new-in...

http://www.imagemagick.org/Usage/resize/#liquid-rescale

Hmm, I knew it was implemented in a bunch of plugins or dedicated software like PS. However I am surprised it's not found all over the place by now as part of default apps / preview / OS builtin software / img tag options, etc.

Also none of the implementations I have tried (last time I checked, not up to date on this) were as slick / realtime / interactive as the SIGGRAPH demo.

Not at all. I have a pretty cursory knowledge of Photoshop and imagemagick (whenever I use either one I have to constantly google things).

Seam Carving is available in GIMP with the "Liquid Rescale" plugin.

$ apt install gimp-plugin-registry

GIMP->Layer->Liquid Rescale