Now it depends against new version. Old code can be removed

src/windowsystem/windoweffects.cpp

@@ -150,12 +150,6 @@ void WindowEffects::slideWindow(WId id, KWindowEffects::SlideFromLocation locati
     }
 }
 
-#if KWINDOWSYSTEM_VERSION <= QT_VERSION_CHECK(5, 61, 0)
-void WindowEffects::slideWindow(QWidget *widget, KWindowEffects::SlideFromLocation location)
-{
-    slideWindow(widget->winId(), location, 0);
-}
-#endif
 
 QList<QSize> WindowEffects::windowSizes(const QList<WId> &ids)
 {

src/windowsystem/windoweffects.h

@@ -37,9 +37,6 @@ public:
 
     bool isEffectAvailable(KWindowEffects::Effect effect) override;
     void slideWindow(WId id, KWindowEffects::SlideFromLocation location, int offset) override;
-#if KWINDOWSYSTEM_VERSION <= QT_VERSION_CHECK(5, 61, 0)
-    void slideWindow(QWidget *widget, KWindowEffects::SlideFromLocation location) override;
-#endif
     QList<QSize> windowSizes(const QList<WId> &ids) override;
     void presentWindows(WId controller, const QList<WId> &ids) override;
     void presentWindows(WId controller, int desktop = NET::OnAllDesktops) override;