I think it would be a small, but convenient addition to show the name and version of the distribution the user is running aligned just to the left of the "All Settings" button.
The distribution info goes into the Xfce About dialog. Here's the code: https://git.xfce.org/xfce/libxfce4ui/tree/xfce4-about/main.c?id=4134b53f680baa65cd6e110168667f325c93a0a2#n481 Usually it goes by "distributed by Debian", "distributed by Xubuntu" or "distributed by Arch Linux". None of them append their version, which makes a lot of sense in Arch's case :) I don't know, but the same package can be used for more than a version of distribution, if so that's the reason they avoid to include the version.
I think there might be a way to get this information programatically... similar to how GNOME and Pantheon do it. Need to do some research to figure it out.
The new xfce about dialog contains the distro information. I would not spam every dialog with this information to be honest. It's not that relevant that you *always* need to see it...