users wanting to shutdown should be in the _shutdown group since OpenBSD 7.4

see https://cvsweb.openbsd.org/cgi-bin/cvsweb/src/etc/group?rev=1.99&content-type=text/x-cvsweb-markup

carried as a port patch since a year and a half https://cvsweb.openbsd.org/cgi-bin/cvsweb/ports/x11/xfce4/xfce4-session/patches/patch-xfce4-session_xfsm-shutdown-fallback_c?rev=1.4&content-type=text/x-cvsweb-markup