The Origin of X-to-Close in UI Design Posted by riveteye on Monday February 10, 2014 @08:58PM from the medium.com dept. https://medium.com/solve-for-x/417936dfc0dc ( 1 comment | 3 points | on HN )
See pg. 39 of this .pdf file http://bitsavers.trailing-edge.com/pdf/microPro/Wordstar_3.3...
"X" is used as the "exit wordstar" command menu option.
This was WordStar 3.3, in 1983. WordStar existed for far earlier than 1983, and likely always used "X" as the "exit wordstar" command.