First I was like
[~]$ chmod +x Subsurface-18.104.22.1685-x86_64.AppImage
Gtk-Message: GtkDialog mapped without a transient parent. This is discouraged.
/tmp/.mount_WQ90Ng/usr/bin/subsurface: error while loading shared libraries: libselinux.so.1: cannot open shared object file: No such file or directory
Maybe remove Arch or make it so that AppImage identifies when it's being run on Arch and explains that one has to get libselinux from AUR unfortunately.