To every smoothbrain that responded thusly, if you only use a word in private amongst certain other people, then it feels a lot like we both know that’s a word you shouldn’t use.

No, any euphemism people invent to talk about things that are negative will inevitably end up as slurs, because people view these things as negative. It's inevitable. Even when they invent inscrutable acronyms like DAMP or ADHD, people will use them as slurs when they denote things that are undesirable.

This is not an example of that though, since "retarded" is not a slur in WSB, it's the opposite.