It used to be that "modern" or "improved" icons had more colors, drop-shadows, 3D shading, more photo-realistic, etc.
Now apparently we've come full circle. Now everything is super minimalistic with no more than 4 colors used (sometimes even TWO colors, with white being one of them), and NO details.
Looking at my desktop on the PC, I note that the few older programs that I have on it have very detailed icons. As you note, they are multi-colored, more realistic looking, 3-D, etc. The newer programs are plain.
The first "plain" icons I remember seeing was in one of the versions of Microsoft Office. Since then, the icons have gotten uglier and simpler as time passes.
I think the reason for this is that "Millennials" really don't have any imagination. Most of them don't
read anything not on a "device". Their whole lives had been planned for them until they reached the age of majority. They can't do things on their own; everything has to be a team project so the end product is always the lowest common denominator. Thus, they have great difficulty in imagining what an icon might really look like for a program so they just draw something simple.