MarsSeed [1] filed a deletion request for qt5-backgroundprocess [2]: Discontinued third-party Qt5.9 library from 2017. Repo is archived. [a] Nothing depends on it.
[a]: https://github.com/Skycoder42/QtBackgroundProcess [1] https://aur.archlinux.org/account/MarsSeed/ [2] https://aur.archlinux.org/pkgbase/qt5-backgroundprocess/
