The reason behind the 4:3 low resolution is outside of scope for ray tracing or performance overall: most of pro players found out that widened picture and thus player models are easier to target precisely in motion (AFAIK the phenomenon hasn't being studied properly) and you're limited by vertical resolution of the screen thus 960 is the upper bound for a common 16:9 1080p panel.
As for lowest settings, the CS:GO don't have options to put the game into flat panel mess that is professional Q3: https://www.youtube.com/watch?v=Q3ui0hz6sm0