the basic point is that it doesn't fucking matter if you're offended and, often times, that's actually the intent. i have a pretty big vocabulary. i get to decide what words are appropriate in how i express myself, not you. and you have a lot of fucking nerve thinking that your feelings are more important than my free will. you can go fuck yourself.
the word is best used to express a certain kind of extreme stupidity. stupidity exists in a hierarchy. something that is only a little bit stupid can be described using a term like dumb. then, there's something moronic, which is more stupid than something dumb. that which is utterly fucking completely retarded is at the very top of the stupid hierarchy.
it shouldn't be used haphazardly, admittedly. it should be reserved only to describe the most stupid of the stupid.
it really has nothing to do with having a genetic defect like autism or down's syndrome. people with genetic defects may act retarded sometimes, and sometimes they may not. generally, that description, to the extent that it may be accurate, would come without the contempt directed at somebody that does not have a genetic defect. you would generally not use the adjective retarded to describe somebody with a genetic defect, unless they are being retarded, which they might be.
but i'm not going to be prejudiced about my hierarchy of stupid or assume that somebody is a retard just because they have a genetic defect.
the term is used most appropriately and most accurately in english usage to describe behaviour that is at the top of the stupid hierarchy, by people that do not have genetic defects, and therefore should be expected not to behave that way, and can be fairly treated with derision and contempt for doing so.
the word's etymology is latin, likely french, and means slow, delay or late - initially, as in dim-witted, or unable to keep up. it was adopted for medical usage, but the use of the term in english long predates that. i could use the latin verb or translate it. there's no logic in yelling at me for using the latin, and thinking it's ok to translate it, but that seems to be the convention amongst the ingsoc newspeak language police.
people that have genetic defects shouldn't assume the word is targeted at them. it generally isn't, and the history connecting the use of the word to the specific context of genetic defect is rather recent. it's just a latin translation for "slow witted".
people that believe that language is something for individuals to decide on, and not something to be determined by centralized bodies, will rightfully push back on any entity trying to remove an adjective from the dictionary. to best express ourselves in the widest variety of contexts possible, we need as many adjectives as possible, and should always be expanding our individual vocabularies, never restricting them.