Clutch safety switch

[Luca]

although a runaway engine must be a very rare problem

Especially when they started putting throttle return cables on the butterfly levers.

It doesn't matter to them if people aren't quick enough to use it, they just need to provide the equipment.

Although I'd hate to do it, I'd pull the clutch and let the engine blow up before I let the bike take me away Toyota Prius style

Another thought I've had (not sure if I'd be quick enough on my feet though) would be to turn on the choke if the engine ever raced away.  As long as it was warmed up, that should cause the thing to foul and bog.

[Dizerens5]

Many British bikes of the 50s (plenty of them still around!) with magneto ignition had an earthing (grounding) pushbutton on the handlebar. Used simply to stop the engine, effectively it was what now would be called a kill switch but I never heard of anyone using it to stop a runway engine. I once did have that problem, on a BSA 650, but it didn't occur to me to push the earthing button. (It was caused by a sticking throttle slide and fixed itself in a few seconds, fortunately!).