We can lift the messagebox to https://invent.kde.org/frameworks/kdeclarative/-/blob/master/src/qmlcontrols/kquickcontrols/private/keysequencehelper.cpp

the widget already has it's own impl https://invent.kde.org/frameworks/kxmlgui/-/blob/master/src/kkeysequencewidget.cpp#L267 as does the kcm https://invent.kde.org/plasma/plasma-desktop/-/blob/master/kcms/keys/kcm_keys.cpp#L285

that leaves onyl yakuake which I think doesn't even need it as it's redundant with the handling by the widget.

Do we actually use the checkForConflictsAgainst in KeySequenceItem anywhere? Quick grep suggests not

By default it is https://lxr.kde.org/source/frameworks/kdeclarative/src/qmlcontrols/kquickcontrols/private/keysequencehelper.cpp#0070