Personally I believe that this rule would be relatively difficult to enforce properly. Most GG maps are relatively small, meaning that you can be in the enemy spawn withing a couple seconds. I'd be surprised if I was the only person who sometimes alt-tabs/takes a hit/runs to the bathroom/runs to grab something while dead. Unless they twitch and move while pretending to be afk there'd be no way to tell if it was a clever ruse or merely serendipity that they come back in time to shoot you in the face. Hell even if they do move slightly while standing still one could make the argument that they were scratching their foot and bumped the mouse.

TL;DR: even if this was a rule it'd be hell trying to enforce it.