Retarded hasn't been a medical term since 2013. Now it just means what it use to mean, that being "delay or hold back."
Why can you get banned for saying something that use to mean something offensive when it doesn't anymore? Any offense taken by it now is just the person being uninformed and thinking that it still means that you are mentally retarded.
Like if we are speaking literally here, calling someone retarded is the same all calling them almost literally anything random.
I'd bet my life you wouldn't get banned if you said, "you are delayed and held back." Even though in reality that all retarded means now.