Most of the forklift manufacturers have in the foreward of their service manuals stating: Un authorized forklift modification is not permitted. Per OSHA 1910.178, no modifications or alterations to a powered industrial truck, which may affect capacity, stability or safe operation of the forklift shall be made without the prior written appoval for that manufacturer, its authorized representative or a successor thereof.
OSHA compliant operator training also states that under no circumstances can any safety item originally installed by the manufacurer be removed or disabled in any way withour prior approval.
Backup alarms are not only required at construction sites, again if installed by the OEM, the only way to get them removed would be a call to that OEM seeking that approval.