Spectacle uses QProcess::startDetached() from Qt 5.10 [1] and KTitleWidget::setLevel(int) from KF 5.53 [2]. Thus it would be reasonable to update dependencies.

[1] https://doc.qt.io/qt-5/qprocess.html#startDetached
[2] https://api.kde.org/frameworks/kwidgetsaddons/html/classKTitleWidget.html#a9972cbbc2c2876ad504dc5ca5ee432d6

Just try to compile Spectacle git/master on some old distribution (Mageia 6). CMake does not complain, but it can be build without patching.