The word was used as a humiliating slur against a vulnerable group of people. This is indisputable fact. It is a word specifically referring to a group of people, and it was used against that group of people to belittle, demean, and humiliate them.

Not something I have disputed, in fact I have made this point repeatedly about the word moron.

The euphemism treadmill stopped.

There will always be a need in language to describe people who are less intellectually capable so I absolutely disagree with this claim. Retard is simply still the word of choice despite efforts to censor its usage.

No one is trying to take away your speech. No one is coming for your words.

Censoring speech is exactly what you've claimed isn't happening, yet it is happening and you are making an argument for the censorship of a word.

Do you use it because it upsets people?

Yes. Because I clearly don't want to have to waste my time on people who are, or are acting, retarded.