SUMMARY
The shortcut `Alt + Shift + Tab` requires setting to `Alt + Shift + Backtab`. However, you can never register a Backtab in System Settings. So Shift + tab doesn't work properly in Task Switcher.


STEPS TO REPRODUCE
1. System Settings -> Window Management -> Task Switcher
2. Reassign the Reverse shortcut by pressing Alt + Shift + Tab
4. Now hold down Alt and Shift, then press Tab twice

OBSERVED RESULT
The shortcut is assigned with `Alt + Shift + Tab`
First Tab press successfully opens the Task Switcher, the second one doesn't move selection backward.

EXPECTED RESULT
The shortcut should be assigned with `Alt + Shift + Backtab`, or KDE should understand that `Shift + Backtab` and `Shift + Tab` are the same thing from user perspective.
The reverse shortcut should work in Task Switcher.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: 
KDE Plasma Version: 5.27.10
KDE Frameworks Version: 5.111.0
Qt Version: 5.15.11
Wayland session
Fedora 39

ADDITIONAL INFORMATION

Manual workaround:

1. Open ~/.config/kglobalshortcutsrc with a text editor
2. Find the faulty shortcut by searching for `Shift+Tab`
3. Replace `Shift+Tab` with `Shift+Backtab`, save the file
4. Logout and re-login