In german medicine it’s still completely fine to use. Retard capsulas for example is medicine that will have a delayed effect after taking them. Someone with brain damage who has issues following a flashlight with his eyes has a retarded response.
The way I saw it, it looked as if you were looking for a backdoor way to use 'retarded.' for a cheap laugh.
I agree, people get upset if you use the terms Negro Baseball League or subnormal children, when those were commonly used terms at the time.
On the other hand, the internet is full of people acting in bad faith. I'd suggest that in the future you don't just post, but take the time to add context. My opinion.