At least in the US fed OSHA regulations and letters of clarifications I have seen, the requirement is to disable the machine when parked unattended, and removing the key if possible is just one more method of insuring "the controls are neutralized" and power is disabled.
In the US [29CFR1910.178(m)(5)(i) ] reg, it states "When a powered industrial truck is left unattended, load engaging means shall be fully lowered, controls shall be neutralized, power shall be shut off, and brakes set. Wheels shall be blocked if the truck is parked on an incline."
So,,, removing the key is one more way to insure the "controls are neutralized", but removing the key is NOT specifically spelled out as part of "the law".
One other thing I note and wonder about sometimes...
"Industrial trucks shall not be altered so that the relative positions of the various parts are different from what they were when originally received from the manufacturer, nor shall they be altered either by the addition of extra parts not provided by the manufacturer or by the elimination of any parts, except as provided in paragraph (q)(12) of this section. " SO how many time have you seen a horn button moved to an after-market button on someplace other than the center of the steer column, or the foot button some manufacturers use, and did you deadline that truck?

Further, any OSHA inspector that insisted a truck had to have an amber flashing light and back up motion alarm, most likely is not a real OSHA inspector, but is a "ringer" trying to sell their (or someone they are in cahoots with) services. They (and we) all know 'if it is there, it has to work', but any OFFICIAL OSHA inspector, that has done more than 3 inspections, should know what the regulation really says. just ask them to point out where motion alarm is stated in the regulation.

And Dan-M, are you confusing "New Hampshire" with "New Brunswick"? ;-) ,
-All- them cold "New" places look like Canada to me, says the guy that grew up in the Fla. Keys, about as far from Canada as we can get and still be in the US "lower" 48.