Rule 3j currently states the following is forbidden:
j. Discriminatory jokes, slurs or hate speech (attacks on an individual or group based on their race, ethnicity, national origin, sex, gender, sexual orientation, religious affiliation, or disabilities)
However, on that last point (or disabilities) I see the word "retarded" being thrown around on a daily basis.
However, retarded has been categorized as a slur, known as the "R word" or "R slur" in most places.
The usage of what I will be calling from now on the R word is looked down upon because it is insulting and/or mocking someone by comparing them to individuals with intellectual disabilities. Which, as a subcontext also says that those people with actual mental disabilities are lesser people.