Who exactly presented the idea that letter "X" should be used for closing/exiting a window or an application?
According to Medium.com,
The first appearance of [x] in GUI design was likely the Atari TOS, possibly influenced by the Japanese batsu and maru conventions. Thanks to a last minute design change in Windows 95, and the mass adoption of Windows worldwide, [x] has become the standard symbol for ‘close’, a symbol that dominates web, app and software design today.
It is stating that letter "x" for closing/exiting a app originated around 1995 but according to my senior, letter "x" for exit was used first in 1980's in wordstar 4 on CP/M OS.
Best Answer
The first GUI use of X for exit was Atari TOS in 1985.
The first PC keyboard shortcut use of X for exit may have been the North Star Horizon OS, which dates back to 1977.
In the early Wordstar versions, X was used for down-line and there was no mouse, obviously.