Depends on why you need to govern and what needs governing. Federally I think ATV destined trucks get a plate welded on the shifter to block off 3rd gear, read some stuff about electrical thingamajigs to stop the engine from going above certain speed but it was deemed too easy to modify
Definitely don't cut the black wire with yellow stripe going to the ignitor. That's the speedometer input, and it's there to keep you safe at 27MPH. If you were to cut it, that would allow the mini truck to travel at unsafe speeds. As to the why: In the US, some states have requirements on the size and speed of vehicles. If it's too small, it's not allowed to be registered. If it's too fast and unregistered, it's not legal. Simple solution was to block off 4th and 5th gears.
the way I interpreted it, is that in order to import and register minitrucks as road vehicles, they need to pass EPA and NHTSA emissions and safety requirements, or be exempt from them. it seems that for EPA, that limit is 21 years (for vehicles with their original, nonconforming engine) and for NHTSA, vehicles older than 25 years old are exempt from the safety requirements (but still need to pass road worthiness if required locally). apparently people were importing vehicles that are older than 21, but younger than 25, and registering them as ATVs or nonhighway/low speed vehicles, or importing slightly younger vehicles and getting their vehicle modified, tested and certified. Most states have speed limitations for ATVs/low speed vehicles around 20-30mph, so to comply with that requirement, they were either governed mechanically, or otherwise prevented from going that fast (such as blocking physical access to select the faster gears)