Oh , come off it. you must be either stupid or a pre-teen if you thought that calling people “retarded” wasn’t offensive.
Personally I don’t find it offending when random people around here call me something like that.
I don’t feel that way, either, but that was never the point. Retared as a slur is not very offensive because it would be particularly insulting to the recipient to be called mentally handicapped, it’s easy to shrug that off, it is offensive because it devalues mentally handicapped people. But I don’t expect you to understand this.