I was happy to see that in Plasma 5.21, all windows remembered their size & position. That was using X11.
Now under Wayland, I see that windows have started not to remember their geometry:
* Dolphin always starts at 0, 0 in a small window
* Same for the KDE Menu Editor
* Same for Konsole
* Krusader remembers its size, but the position is again 0, 0
* PoliciKit KDE1 shows up in the bottom right corner
Linux/KDE Plasma: 5.21.1
KDE Frameworks Version: 5.79.0
Qt Version: 5.12.5
Yes, that feature only works in X11. The mechanism used there involves the window positioning itself, which KWin then honors. This does not work on Wayland because windows are not allowed to position themselves at all, as a matter of protocol. So any position remembering feature needs to be implemented in KWin itself. That makes this a duplicate of Bug 15329 which is really only implementable on Wayland to begin with.
*** This bug has been marked as a duplicate of bug 15329 ***