the Super_L key is permanently bound to the app menu with no apparent way to un-set it, not from system settings and not from the app menu keyboard settings, and it doesn’t matter which app menu is being used
searching the keyboard shortcuts for Super_L or meta reveals several shortcuts where this key is used in combination with another key(s), but none that use only this key
i think it was the 5.26.2 update that caused this
KDE Plasma Version: 5.26.2
KDE Frameworks Version: 5.99.0
Qt Version: 5.15.6
Kernel Version: 6.0.6-1-MANJARO (64-bit)
and yes, Super_L has, by default, been bound to the app menu for a long time, however one could un-set it in settings > shortcuts up until 5.26 (or 5.26.2)