I know this isn’t popular nor what most people would agree on, but to me retro is pretty much synonym with 8 bits (maybe some early 16bit) and the arcades that were around at the time. May have to do with me being old, but I think it is more because I felt a shift in look & feel, from sprites, pixel art, side scrolling, moving room to room, etc. to a more “modern” feel, with 3d models, etc. of course now we have a bajillion more polygons, shaders, ray tracing, but it is just a change in degree of the same thing.

Of course you still get games nowadays that look retro, but it is done out of artistic choice, not necessity. The constraints of the old platforms led to truly inventive solutions.