Impact sockets are exactly as you describe them (This Ingersoll Rand Tool Tip provides info.)
Don't let the idea of the impact socket being more pliable fool you. The main idea of the impact socket and the reason why it can flex is to handle the extreme, sudden torque which is applied through the impact gun. This flexibility will not affect the outcome of the torque which is being applied and measured by the torque wrench. The reason for this is because if the socket head flexes during the torque, the torque wrench doesn't register the give (it equates it to the same amount). Since the socket is attached to the head of the torque wrench, it's considered part of the torque wrench (the head of the fastener could be considered the same). You may twist the handle more, but the needle won't swing any further (or the click won't happen or the meter won't register any more torque applied).
The flexing of the socket is so minimal, this should never be a worry. For that matter, a regular socket is going to flex as well, but we don't worry about them. It will just flex a little less than an impact socket, is all. Again, the difference is going to be quite minimal and nothing to worry about.