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.
Origin of letter X for close icon
history;user interface
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. 