Upgrading from 2015 to 2016 has been a nuisance.
None of my plugins are there. The suggested solution was to look at plugins previously installed from Extension Warehouse. Since all my plugins don’t come from Extension Warehouse it was further suggested that I go to the SketchUcation plugin store. However, SketchUcation doesn’t automatically keep track of plugins downloaded. More importantly some of my plugins came from other sources than either of these two.
The appropriate place to keep track of plugins is within SketchUp itself. And when upgrading you should be given the option to 1) install, 2) not install, 3) update each of your plugins. I understand that plugins may not be compatible with the latest version of SketchUp, hence the option to not install (with a warning) or to update.