This is just CS:GO finally evolving from the CoD brown-tinged visuals of 2007-2013

No? There's more color, but it's reasonable CSGO always had a kind of dull color grading/textures, though updates changed that A bit later on. A lot of players turn up saturation themselves for the (at least perceived) benefit of visibility so that might be what you saw, you also see CS players playing in 1024x768 stretched still because that's how they always played before.

They definitely gave it that trendy sunny slightly hazy day look. I'm not complaining it's a much nicer aesthetic than the original release of CSGO which felt like a Seattle afternoon in the middle of the desert.

The saturation doesn't look any different to me when on my own machinr, the streamer may have changed their color settings on their GPU for a number of reasons. Visual clarity can be important for seeing targets quickly, as well as maybe wanting to stand out when scrolling in a feed so viewers see their stream stick out?