I also lost the Show Desktop widget, although as I never made use of it, it was no great loss, but in consequence I didn't search the Forum and get the reference to kde-plasma-addons. However, one favourite widget, the Fuzzy Clock also vanished, and searching 'fuzzy' and 'clock' in Synaptic and the Forum didn't locate any relevant results. Panel Options/Add Widgets listed a (different) 'Fuzzy Clock (Javascript)', but this crashed with a Javascript error on dragging it to the desktop. Eventually, through discussion on the kde.org support forum, I found out about kde-plasma-addons, installed it and was reunited with my old friend.
Obviously this isn't a universe-shattering problem, but if the description of kde-plasma-addons in Synaptic had included a list of the contents, my original Synaptic search would have given the information I needed at a much earlier time. Some portmanteau packages do have this sort of list, and I wonder if it might be done more regularly to provide a better search facility in future?
BTW, congratulations to all who stitched the KDE upgrade to 4.8.3 into the normal Synaptic upgrade framework. In my case it ran like a dream and the forum provided all (well, nearly all!) the aid I needed to get everything running smoothly.