After last update on my Fedora release 24 installation, even after I select "enable editable accelerators" in the xfce4-appearance-settings window, I am no longer able to remove the Alt+<number> shortcuts in xfce4-terminal which are used by default to switch tabs. This interferes strongly with irssi which uses them to switch windows. Would it be possible to either re-enable the edition mode or provide another way of setting those shortcuts?
I have the same problem with xfce 4.12 and terminal 0.8.1 using Arch Linux. Editing the accelerators works in Thunar, so it seems to be a problem specific to Terminal. As a workaround, I was able to change the shortcuts by commenting out and adjusting the respective entries in ~/.config/xfce4/terminal/accels.scm.
Unfortunately, gtk3 does not support editable accelerators. Since xfce4-terminal is a gtk3 app, this feature isn't available anymore. But Martin is right, all terminal accelerators can be changed via ~/.config/xfce4/terminal/accels.scm.
The valid values for the accelerator keys in accels.scm are found here: https://git.gnome.org/browse/gtk+/plain/gdk/gdkkeysyms.h (Just ignore the GDK_KEY_ prefix).