Yes, the horn is a safety device. Not just applicable to Toyota brand either.
All safety devices need to be maintained in the OEM configuration to assure the operators can habituate themselves to the exact location of the controls for the safety device.
Moving a horn button to a location different than the OEM placement removes the reflex action taken (by the operator to sound the horn) from being the correct action when a situation suddenly pops up where the horn should be sounded.
This becomes even more critical in a large fleet of forklifts where an operator might work for weeks without driving the same forklift twice.
Moving the location of a horn control button?? Might be "legal", but it is not smart.
Now, to be honest about my somewhat unique position, I will admit that all our forklifts do in fact have a second horn control button/switch mounted on the dashboard near the key switch. That second button is there as a supplemental horn button or auxiliary horn button to assure the truck horn can still be used until our shop addresses the report of an OEM steering wheel horn button inoperative. Our policy is that BOTH horn buttons must be in working order at all times, or place the truck out of service.
Horns not working is as bad as brakes not working with regard to how we address the issue at our shop.
We want a truck with an inoperative horn placed out of service and presented to our shop ASAP when it is discovered.
Some operators are good about complying, others would rather pass the buck to the guy on the next shift.
We have assigned operators that do a daily inspection of all the forklifts to augment the required pre-trip inspection the individual operators are "supposed to do".
Our assigned inspector/operators do in fact report more defects than the regular operators, and that is why we have a redundant plan in place.
OK, rant over.