Several important things have changed since SU8.
First, user plugins are no longer installed in Program Files folders. They are installed in User Data folders. But due to the next points I won’t tell you where.
Second, part way through the SU8 lifetime, a new way to install extensions was added to the Preferences panel. In addition, the Extension Warehouse and sketchUcation plugin store matured significantly, allowing you to download and install extensions directly. These three ways are now the supported way to instal extensions, not by copying the Ruby files.
Third, as of SU 2014, the version of Ruby used by SketchUp for plugins changed. This was a major upgrade, and many old plugins broke. If you simply copy old plugins from SU8 to SU2016 a significant fraction of them will fail, potentially causing SketchUp to crash on launch.
So, the strongly recommended practice is to obtain new copies of the extensions and plugins you need and to install them via the Preferences panel, Extension Warehouse, or SketchUcation plugin store systems.