Merci bolivari.
Je voulais donc procéder ainsi, mais il s'avère que des logiciels sont dépendants (et la flemme de réinstaller) :
$ flatpak remove org.fedoraproject.Platform
ID Branch Op
1. [✗] org.fedoraproject.Platform f35 r
Error: Can't remove org.fedoraproject.Platform/x86_64/f35, it is needed for: org.gnome.Calculator, org.gnome.Calendar, org.gnome.Characters, org.gnome.Chess, org.gnome.Contacts, org.gnome.Evince, org.gnome.FileRoller, org.gnome.Logs, org.gnome.Maps, org.gnome.NautilusPreviewer, org.gnome.Screenshot, org.gnome.Weather, org.gnome.baobab, org.gnome.clocks, org.gnome.eog, org.gnome.font-viewer, org.gnome.gedit
error: Failed to uninstall org.fedoraproject.Platform: Can't remove org.fedoraproject.Platform/x86_64/f35, it is needed for: org.gnome.Calculator, org.gnome.Calendar, org.gnome.Characters, org.gnome.Chess, org.gnome.Contacts, org.gnome.Evince, org.gnome.FileRoller, org.gnome.Logs, org.gnome.Maps, org.gnome.NautilusPreviewer, org.gnome.Screenshot, org.gnome.Weather, org.gnome.baobab, org.gnome.clocks, org.gnome.eog, org.gnome.font-viewer, org.gnome.gedit