Chasing up deprecations prior to Qt6, I find that Qt::WA_ShowModal and Qt::WQ_GroupLeader have been documented as deprecated since 4.5.1; however, the QWidget::windowModality that they say to use instead is implemented in terms of setting these properties; and there is severely non-trivial code in the property management implementation, that's triggered by setting them.
These properties should be either eliminated (preferably in time for Qt6 FF) or un-deprecated and properly documented (in qnamespace.qdoc).