One phrase I've heard in colloquial conversation and in some media is "short bus", which is offered as a synonym for the r-word. Sources on this word (CW: these sources may not abbreviate the offending term to "r-word"):

• Wikipedia entry.
• Online Slang Dictionary entry.

I think if we're going to highlight the r-word (and one would be right to do so), we should offer this as a synonym in some place. It's a term of derision people use to sidestep actually saying the r-word, but has similar, hurtful outcomes.