if this is a thing, maybe it would make sense to actually try and remove the KWindowShadows upon destruction?
Something like connect(newshadow, &QObject::destroyed, this, [this, widget] { _shadows.remove(widget); });

We have a connection to destroyed for the widget which isn't reached when a window is destroyed on QApplication teardown, so I think the same might be true for the window, I would still prefer this over a QPointer.

if this is a thing, maybe it would make sense to actually try and remove the KWindowShadows upon destruction?

Yes, that's a better idea.

so I think the same might be true for the window

No, it works fine.