I play 1024x768 stretched. Only shows up 4:3 in the screenshots. I have a 144hz monitor and play esea etc on 128 tick servers so I always need above 128/144 fps or else there's a disadvantage. I did try playing 16:9 1600x900 for a while but genuinely felt my aim wasn't as good. It's also easier to control spray patterns on lower screen resolutions. It's nice having a small area to view, where your minimap, inventory, ammo etc is all close together and easy to read quickly.
Most pros use 1024x768 from what I've seen, mainly blackbars as well. There are a few that use lower for example ScreaM uses 800x600, then some use higher for example f0rest uses 1600x900 and Swag uses 1920x1080