Incomplete combustion is the main reason for carbon deposits.  For our engines, this happens mostly during low RPMs or low engine loads.  Fair to note that in the Kubota engine, fuel injection happens in a pre-combusition chamber which goes a long way to preventing carbon build up in the first place.

In order to remove excessive engine carbon build-up that are not removed during normal cruise RPMs, you have to raise the temperature of the cylinders and this is why some would advocate periodic WOT.  To remove exhaust system carbon build-up, the increased temps and gas volume might do something, but since our exhaust systems are water-cooled, only a short segment of exhaust would really benefit.

For me, going to WOT is something I rarely do, and only then for short durations (a few minutes) in a safe area as a way to verify that I can in an emergency, or diagnose a condition.  For a young engine, extended WOT operation might be OK, but for an older engine the cooling system, bearing clearances, tappet gaps, injector timing, transmission gear lash, etc. might be less than optimal and therefore caution is advised.

Net-net my opinion is that operating at normal cruise RPM and regular oil changes gives you sufficient cleaning benefits and therefore going to WOT for cleaning benefit is unnecessary.  Murphy's law - if it ain't broke......