A discussion came up some time ago in the Linuxquestions Slackware forum:
in the 'Xfce for Slackware Users' discussion. It seems there are a few
who dislike the icon tool tip pop-ups in the Xfce panel. Back when, there
was a settings that would toggle this functionality. In Xfce 4.16 there
isn't...
...or is there?

If you enable desktop compositing (Settings -> Window Manager Tweaks ->
Compositor), you can set the tool tip to transparent, effectively making
it disappear. Simply edit the CSS in $HOME/.config/gtk-3.0/gtk.css to set
the tool tip opacity to zero and restart Xfce.

@import 'colors.css';
tooltip {
    opacity: 0;
}

Of course, this means that any program using this GTK3 CSS style will
also have the tooltips become invisible.