Cano:

ANSI/ITSDF define the load backrest extension as a removable device. Removing it for your purpose would not be considered a modification.

"load backrest extension: a removable device that increases the load restraining area beyond that provided by the load backrest. " (ANSI/ITSDF B56.1-2005, Safety Standard for Low Lift and High Lift Trucks, Part IV, Glossary of Commonly Used Words and Phrases.)
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ANSI/ITSDF require a load backrest extension only when the load presents a hazard.

4.5.2 Load Backrest Extension. If the type of load presents a hazard, the user shall equip forklift trucks with a vertical load backrest extension manufactured in accordance with para. 7.28. (ANSI/ITSDF B56.1-2005, Safety Standard for Low Lift and High Lift Trucks, Part II, For the User.)

5.2.18 A load backrest extension shall be used when necessary to guard against a load, or part of of it, from falling toward the operator. (ANSI/ITSDF B56.1-2005, Safety Standard for Low Lift and High Lift Trucks, Part II, For the User.

7.28.1 The load backrest extension, if provided, should have height, width, and size of openings sufficient to minimize the possibility of the load falling toward the mast when the mast is in a position of maximum rearward tilt. ((ANSI/ITSDF B56.1-2005, Safety Standard for Low Lift and High Lift Trucks, Part II, For the Manufacturer.)
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OSHA requires the use of a load backrest extension when necessary.

1910.178(m)(10) - A load backrest extension shall be used whenever necessary to minimize the possibility of the load or part of it from falling rearward.
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Google: ITSDF.ORG for a free download of the ANSI/ITSDF B56.1 standard.