The “brushed metal means hardware” [0] concept was always something I thought would’ve worked great in practice. Of course apple never really committed to that, and made any window they could look like a slab of aluminum for a while.

But the idea of making a subtle signal that this thing you’re interacting with will have external effects, in the age of MP3 players and digital cameras, was smart.

[0] https://daringfireball.net/2004/10/brushedmetal

I've seen speculation that we are slowly returning to a less flat, more depth-oriented design trend based on what Apple showed in the Vision UI demo (skip to 15:13 to see all the components): https://developer.apple.com/videos/play/wwdc2023/10076/

Notice the shadows inside of the in the inputs, and the 'shine' applied on the bottom. Web designers from the 2000s will remember all this stuff. What's old is new again!