When we made the switch to the dockable palette, we ditched this dialog box, as it was far too large.
You can run a schedule with the Entire Palette option to see which plants have which symbols. And using the Match Plant button as your primary tool, note that it will highlight the selected plant in the docked palette; further, you can use the Q/E keys to toggle through.
So with all of that, we just felt it was not necessary to bring back a big clumsy dialog box. But we do agree it had its merits.
Note interestingly, for all other portions of the software that never had that dialog, the feature has never been requested. So for Site Amenities and Irrigation, they have never had a symbol preview.
--J