There are plenty of communities with some of the feel of the Amiga scene. I'm most familiar with the Pandora community here, and while it has a few notable homebrew games it mostly gets ports these days. Then there's the dingoo scene, and the various knockoffs of that, the hacked hardware scenes and so on.
Sadly, I think demos are a dead art in the main. Kids and young adults are the only people with enough free time to make something so complex for free, and these days there are far easier ways to spend you time than getting to know the nitty gritty of all the APIs. But there are still demos being released from the scene compos, looking at pouet.net, and still quite remarkable C64 events and so on being organised. I think those are two different things though - C64 hacking depends on finding undocumented features that would be patched away on a newer connected system. But any API will have corner cases where they do things quicker and differently to the way you'd expect, but are still legitimate parts of the API, just in my experience they aren't nearly as interesting.