This was a kludgey hack that never managed to land upstream, yet utterly dominated (for a brief moment) the headspace of the early linux desktop. It's funny how quickly things were moving at the time. In the mid 90's, GUI design elements were still in their infancy. Even basic stuff like "what do windows do?" was in flux. Traditional X window managers hadn't settled on anything like a regular usage model: twm was sti…
The close button has always been there, you double clicked the top left menu button. That worked all the way until Microsoft started redoing window decorations in desktop mode with Windows 8.1, and even for a short period after. This was also copied into other X window control styles. Even today, a Motif replicates the Windows 1.0-3.11 top-left menu+close button.
Yeah, yeah, I know CUA allows for a window close. No one knew. I worked IT at the time (as did lots of us here in our youth I'm sure) and was constantly teaching and re-teaching this trick to the poor people trapped with their CUA environments.
But suddenly with Windows 95 you could see how it worked.
[1] Even if we knew in our bones, c.f. this very discussion about the popularity of a cloned hack on Linux, that it was the Right Thing.