I had noticed the same thing. But then i noticed something even stranger: Inside the Plugin folder (1) lies an executable: "GrowlExtras.ITunesPlugin.exe". If you manually start this, the plugin works again, as it should.
Oh, and don't worry about the error message. Click "don't notify me again". That's what i did.
FYI: I'm on iTunes 12.5.1 64bit, on Windows 7 64bit.
(1) in my case: C:\Program Files (x86)\iTunes\Plug-Ins\GrowlPlugin