The parallels between the ableist slur and the racist one run deeper than your argument seems to acknowledge. The word “retard” actually does have a specific history and a specific target. It wasn’t just common vernacular - it was a medical diagnosis.
The reason medical practice has completely abandoned its use is the same reason society should abandon it - it has a history of exclusion, prejudice, and measurable social harm.
By using an outdated (and objectively terrible) diagnosis as an insult for people who we deem intellectually inferior, we continue to associate developmental and behavioral disabilities with being inferior, and perpetuate the systemic and systematic injustices that some of our most vulnerable population still face to this day.
What do you mean by “designed”?
The euphemism treadmill is a known issue. The reason this ableist slur is offensive is, yes, because it is the most recent turning of the wheel. It is the word used in living memory to both refer to patients with poorly understood medical conditions and as an insult to people deemed intellectually inferior.
There is no designer of words. What matters is how they are used. The word “retard” was used to cause harm. It was used by people to broadly and injustly categorize a group of vulnerable individuals by genetic and environmental conditions outside their control.
It was used as a vicious insult by peers and authority figures, it was used in schools and workplaces, it was used by doctors and parents. It was used - yes - to be cruel and humiliating. Of course it was.
Nobody designed the word to cause harm. But anyone who remembers the schoolyard knows that there are countless kids with very real conditions that were mistreated and misunderstood by professionals, parents, and peers. Some may have used the word in good faith. But many more used it in bad faith. They used it as a tool to be cruel and humiliating, and of course they used it on children and adult who could have been diagnosed with a wide variety of very real (and sometimes treatable/manageable!) mental and behavioral conditions that we are still barely scratching the surface of to this day.
It caused harm. It continues to cause harm. And the people who were and are harmed by it are still alive today. Those children grew up to be adults.
People don’t choose to be offended. People are offended by any number of things for any number of reasons. It’s usually not a conscious choice. It’s often a result of injustices experienced or injustices witnessed. In this case, it’s because many of us remember when people used the word “retard” specifically to be cruel and humiliating to vulnerable people.