The stainless steel bracket is more durable and less likely to corrode. The new legislation specifically addresses the problem of rotting wood frames. You are not required by law to install a stainless steel bracket if your window aircon is not supported to a wooden frame. However, you should check to ensure that your aircon is securely fixed to the aluminium or metal frame. If it falls off because it is not properly fixed, you could be sued if the aircon causes injury or damage.
Many buildings have concrete ledges or casings specifically designed to hold the aircons installed.
If your aircon sits securely on a concrete ledge or inside a concrete or steel casing specifically designed to hold it, then there is no need for you to retrofit your aircon. The aircon will not fall to the ground with the ledge or casing. However, there are some cases where the aircon is not sitting on a ledge but is located above a ledge. In such cases, you have to make sure that should the aircon fall, it would not topple over or slide off the ledge after it hits the ledge. This can happen if the aircon is not located near the ledge or if the ledge is not wide enough.