Since upgrading from Xubuntu LTS 16.04 to LTS 18.04 (using xfce4-panel 4.12.2), some SVG icons are not retrieved by the Launcher icon chooser; for example several instances of preferences-system-time-symbolic.svg. This bug has been independently confirmed and discussed for another XFCE distribution; NixOS 16.09beta, see: https://github.com/NixOS/nixpkgs/issues/18536
Moving to the appropriate product.
I think I found the problem: exo-icon-chooser doesn't display icons that are symlinks. This happens both with svg and png. This used to work, at least in Xfce 4.10. Quick way to reproduce: in many icon themes, editcopy is a symlink to edit-copy in all directories, for example: /usr/share/icons/Papirus/22x22/actions/edit-copy.svg /usr/share/icons/Papirus/24x24/actions/edit-copy.svg /usr/share/icons/Papirus/16x16/actions/edit-copy.svg /usr/share/icons/Papirus/22x22/actions/editcopy.svg /usr/share/icons/Papirus/24x24/actions/editcopy.svg /usr/share/icons/Papirus/16x16/actions/editcopy.svg so just search "copy" in the icon chooser, and you will only see edit-copy instead of both editcopy and edit-copy. This makes it very hard to find the right icon as many themes have lots of symlinks.
Actually the correct component is exo, as it contains the icon chooser dialog.
*** This bug has been marked as a duplicate of bug 10126 ***