Back in the day GM did install a drain plug in the tranny pan , Only one BIG problem , In thinking one was changing the oil they would drain all the tranny fluid , by mistake , then put 5 quarts of oil in the engine . So now you have your car with 10 quarts of oil and no tranny fluid . BIG ASS MESS . Now is that the reason they stopped with the drain plug in the tranny ? I have no idea but you know it had to happen .