[yorkie_chris]

So, you downshift through the gears to slow you down. There's 2 possible sources of damage.

Not blipping the throttle and just banging down through the gears, this is never going to be good as it makes crunchy noises. Crunchy noises in gearboxes are generally expensive.

Downshifting at 10krpm, this is going to rev the balls off the engine with the possibility of breaking things due to the extra load on everything, for example the camchain will have far more inertial forces on it than normal, you risk valve float, and piston speed is higher, giving more stress on conrods.
All these things are possibly very catastrophic and instant failure.

So, IMO, do it smoothly, and don't try to get it to rev higher than the limiter and it's fine.