The documentation states there is (PNG) icon support for x-data.
There is the obvious cut’n’paste error in the path there. But other than that there is seemingly no support for icons in x-data whatsoever. I’ve tried both PNG and SVG, restarted the server etc, but no luck so far. With the new compressed shortcut bar in 6.15.x that shows an icon for the settings and access sections in the content editor, you would think this to be a perfect opportunity to be able to do the same in your own apps.