ETPro added a b_pronedelay cvar that made you immobile for a period of time whilst standing from prone. It doesn’t stop you shooting whilst going prone, it just stops you moving left / right while standing up again, and the delay period most commonly used is 600ms. It’s set at 0ms in leagues, i.e. not used at all.
Proning was a big issue in ET a few months ago, as the problem essentially is that the animation of a player going prone doesn’t match what actually happens. In other words the player’s hitbox is NOT where you see it during the proning motion. ETPro did also fix the hitbox so that it goes through a stand -> crouch -> prone sequence (and prone -> crouch -> stand when standing up I believe) but it still remains that the animation you see when someone goes prone doesn’t match what actually happens.
Some would argue this is insignificant anyway as the hitboxs rarely match the player model as it is. The head hitbox, for example, is on a player’s shoulder when they’re strafing. ETPro may have also fixed this in the next version, which uses new server-side player model hitbox detection. That will be a blessing for people like me who simply can’t come to terms with NOT aiming at what I see.
Proning however isn’t as big a problem now as it was. Most proners have realised that it may make their opponent miss the first couple of shots after they’ve proned, but it puts them at such a disadvantage that it’s rarely worthwhile.