So if so if Jet Set Radio was released June 2000, you can look for related papers a couple years before to see if new techniques were appearing. And, in fact, they were!
Disney paper (1998) on texture mapping for cell shading (the color of a cartoon):
https://media.disneyanimation.com/uploads/production/publica...
NYU paper (1998) applying outlines to 3d objects (the black outline of a cartoon) :
https://mrl.cs.nyu.edu/publications/npr-course1999/hertzmann...
It only looked like crap in its era because carts were expensive compared to CDs. Which is less of an issue now.
Also the hardware antialiasing and overuse of fog didn’t help its case. Thankfully the former can be fixed either via hardware mods or emulation.
I’d be still be interested to see if those demos you saw were full games or not. I’ve seen a lot of cool effects in games get abandoned because they didn’t scale well to a fully fleshed out game.