It's has been a slur for decades. It's hard to precisely say what decade. My experience is from back in the 60s at my Jr high school. My Jr high had a special education 6th grade in it and the shitty little kids would call those in that class the r-word. The word has bothered me for decades.
Further research shows when they were changing on a more national level so the use of the slur I heard in the 60s may have taken another 10-20 years to evolve as an everyday slur.
This from the archived link below.
"In 1987, the group changed its name. Years of the use of “idiot,” “moron,” and “imbecile” as common insults had already inspired a few name changes; this time, the organization went with a progressive, respectful new term that had been introduced in the 1960s, becoming the American Association on Mental Retardation.
It took only two decades for popular culture to drag “mentally retarded” through the mud enough to prompt AAMR to change its name again. In 2007, it became the American Association on Intellectual and Developmental Disabilities.
Another group, the Association for Retarded Citizens of the United States, got around to dropping “retarded” from its name in 1992, but since it had been known as “The ARC” for many years, that group wisely made “The Arc” it’s official name, sans acronym. It won’t have to go through another name change.
“Mentally retarded,” once the respectful phrase that replaced the insults, is now the new term to hate among people with intellectual disabilities (the preferred term today) and their families and supporters"