Under Windows, as it seems, plugins are now moved into a different location when reinstalled. They now go to:
"C:\Program Files\Common Files\Adobe\Plug-Ins\CC" (rather than "C:\Program Files\Adobe\Adobe Photoshop CC 2015.5\Plug-ins").
I reinstalled three or four of my plugins (such as ON1, Topaz, etc.), and they all went into the "Common Files\..." (rather than Photoshop's own) folder. Which is, I suppose, a good thing, since there [presumably] will be no need to reinstall plugins any more upon subsequent Photoshop updates such as this.
However, like others here, I did copy and paste manually the rest of my plugins into the usual "default" location
("C:\Program Files\Adobe\Adobe Photoshop CC 2015.5\Plug-ins"), and they appear to be working fine as well.
P.S.: Just waiting on the PhotoKit Sharpener -- has anybody moved theirs successfully manually?