MarsSeed [1] filed a deletion request for qgis2-crayfish-plugin [2]: Broken legacy version of this plugin from 2018 for qgis v2. But now even the qgis-ltr (Long Term Release) package is on qgis v3, so this package is defunct. There's another, newer package for this, working with qgis 3: https://aur.archlinux.org/packages/qgis-crayfish-plugin [1] https://aur.archlinux.org/account/MarsSeed/ [2] https://aur.archlinux.org/pkgbase/qgis2-crayfish-plugin/