In fact, there hasn't been an actual MPAA rating with the letter 'X' in it since 1990. (And there NEVER was a triple-X--that was a porn marketing ploy.)
It was replaced with NC-17 that year.
To show how mores have changed,
Midnight Cowboy was, I think, the first mainstream movie to be rated X at the dawn of the MPAA ratings system. Nowadays, much of the content in that movie would be considered tame for a cable show, and even some broadcast TV shows. -- RR