They block off the governor on manual shift valve bodies because they use a manually directed oil flow to apply gears.
The valve body is designed to where all it does is redirect flow from one set of internal parts to another set to change gears without any effect from driveline speed at all, so the governor is pointless and therefore blocked off or removed completely.
Probably removed completely if it was professionally built for racing, and in that case the plug would HAVE to be there so line pressure wasn't lost through the open passages where the governor was.