So how would you word it in a succinct, readily understood way? Look up the history of the term if you want to understand the situation. When "mental retardation" was coined in medical literature it replaced far more offensive words. Over time it has come to be seen as pejorative, but it's better than what came before it.

Under federal law, it should have been "intellectual disability" - Public Law. 111-256, §2(b)(2), 124 Stat. 2643 (Rosa's Law) was passed in 2010, which requires the elimination of the term "metal retardation" from use in federal regulations. That makes me wonder if the DOT rules quoted above are from an outdated source.