Control: fixed -1 4.15.3-1 This was already addressed in Thunar 4.15.3-1, which added the gvfs-backends package to its Suggests field. Considering that and also the fact that both Debian Wiki[1] and the official Xfce documentation[2] mention this, it's not within reason to add 30 or so extra dependencies to Thunar. And it's extremely unlikely that the gvfs-backends package will ever be marked as a direct dependency of Thunar.
[1] https://wiki.debian.org/Thunar#FTP_.2F_SFTP_etc. [2] https://docs.xfce.org/xfce/thunar/unix-filesystem#gnome_virtual_file_system
