It is a really interesting trend called the euphemism treadmill. What is basically says is that euphemisms evolve over time into words with more negative connotations. This process is also known as pejoration or semantic change.
If we look back in time to the 50’s, some of the words used can be really surprising because of how offensive they would be if used today. Even turning on the television to an episode of the 1970’s tv series M*A*S*H can reveal some words that would never be heard on television today.
Words originally intended as euphemisms eventually acquire more negative meanings, often becoming dysphemisms. This can be seen in the way we refer to race or disability best.
George Orwell noted this as far back as 1933, showing it’s been ongoing for a long time. An example of this is "lavatory." Now most commonly referred to as a bathroom, washroom, or restroom, lavatory was phased out as it gradually came to be deemed inappropriate.
It stands to reason that the terms we use today, like "mentally challenged" or "special" will take on more negative meanings in the future. Some argue that "special" has already started to go this route, as some consider calling someone "special" to be implying that there’s something wrong with that person.