IMO it's a usability issue. Instead of scanning the UI for a color+shape, there's only a shape. It's less distinguishable from the other icons, so I'll be slower to find it on the screen.
(Similarly, this also bothers me with the PS5 controller. The triangle, circle, square and X buttons no longer have different colors. This might not be an issue if you're already familiar with the button layout, but I bet it makes it more difficult to learn for somebody that is new to the controller.)
Agree, color+shape is important in finding things. I rather like the way thunderbird went recently with the important folder colors. Much better. Moving away from that is a step in the wrong direction.
(Similarly, this also bothers me with the PS5 controller. The triangle, circle, square and X buttons no longer have different colors. This might not be an issue if you're already familiar with the button layout, but I bet it makes it more difficult to learn for somebody that is new to the controller.)