SUMMARY

After upgrading `plasma-workspace` from 5.24.5 to 5.25.1 when starting a plasma-i3 session¹, instead of i3 (as defined in KDEWM environment variable) it starts a kwin_x11 window manager.

Downgrading `plasma-workspace` to 5.24.5 and restarting SDDM fixes the problem.

1: Documented here https://userbase.kde.org/Tutorials/Using_Other_Window_Managers_with_Plasma#System_Wide:_Adding_an_XSession

STEPS TO REPRODUCE
1. Make sure you have installed Plasma and i3wm
2. Create `/usr/share/xsessions/plasma-i3.desktop` with the following content:
    [Desktop Entry]
    Type=XSession
    Exec=env KDEWM=/usr/bin/i3 --shmlog-size=26214400 /usr/bin/startplasma-x11
    DesktopNames=KDE
    Name=Plasma with i3
    Comment=Plasma with i3
3. Restart DM and start a graphics session called "Plasma with i3"
4. Once the session loaded, execute `ps aux | grep "[k]win"`

OBSERVED RESULT

Last command has output (the running kwin_x11 process)

EXPECTED RESULT

There's no output

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Archlinux
KDE Plasma Version: 5.25.1
KDE Frameworks Version: 5.95.0
Qt Version: 5.15.5