I am not sure if it can be called a gta bug . It's caused coz of the way skin hit fails to sync hits during animations(lag).Tbh, most of us do that 'bug' BOTH intentionally and non intentionally.
But the point is, it is not hard to kill someone doing this car entry anim(unless it's a laggy player with 300+ ping), you can time your grenades, chainsaw etc. Also, if make a rule against this, it will be quite a hassle for admins to babysit players 24/7 and everyone would have to record 24/7 to report someone on forum. I'd be completely fine with this if there can be a script against this.