It’s definitely not dead. Some great demos have been released in recent years in JavaScript / WebGL, e.g. using visual tools like cables.
https://cables.gl/
Then wikipedia is wrong. The demoscene is in part about pushing the limits of whatever platform you're on. But another way of looking at the scene is as an art movement based around real-time audiovisuals. The latter is of course a much more ambiguous definition, but there's a certain shared spirit and a defined aesthetic that holds across a lot of different hardware, rendering techniques and formats.
After reading more about it, I think that describes it well. At one point it seems it was very focused on working on limited hardware but that doesn't appear to be a general overall focus anymore. The old hardware demos still exist but most of the modern demos are about making something cool and fun and the fact that it doesn't push modern hardware at all is irrelevant.
The hardware limitations are what makes the demos what they are. If it were not for them, we'd all make pre-rendered animations.
There's a reason why you've got the 4Kb, 64Kb competitions. We want limitations because overcoming them is part of the art.
Lastly I made an Apple ][ demo. I could have learned the shader stuff, load tons of geometry, etc. But I much prefer limitations because I know my peers will check my code and hopefully will be astonished by what I do. (yeah, gratification !!!)
I would even call JS13K [1] demoscene-esque in that its all about getting the most out of work within constraints and limitations. Maybe not a Wikipedia textbook definition but its got that vibe.
The Wikipedia entry on demoscene seems to specify that it's about impressive demos on old computers.