Kubuntu 23.10
KDE Plasma Version: 5.27.8
KDE Frameworks Version: 5.110.0
Qt Version: 5.15.10
When pressing the "Log Out" shortcut or selecting the "Leave" option
I think it should look something like this, or so it did in Kubuntu 18.x:
as discussed here.
On the other hand, it might be the case that the background was removed in later Plasma: in Solus KDE with Plasma Version: 5.27.10 (KDE Frameworks Version: 5.115.0,Qt Version: 5.15.12) tere is no image, but there is a high level of transparency.
But in my case, no matter the theme I use, all the options appear on a dark gray background, with an almost imperceptible transparency. Can that be changed so that a wallpaper be set for this interface or that transparency be increased?
That is a semi-transparent window and as such it is affected by the "Blur" Desktop effect. This can reduce transparency if it's set too strong: decreasing its strength increases the transparency of these windows.
The image at Phabricator discussion linked in question is just the desktop wallpaper seen through a very transparent "Leave" window (that's why we see the panel and the desktop folders)