I don’t mean that it’s not a requirement, but it seems like it’s not actually accepted for the I-751 according to the G-1145 instructions and so would generally be ignored.
Unless the instructions directly from USCIS are incorrect, they state on their website for the G-1145:
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This service is only available for certain forms filed at our four Lockbox locations in Lewisville, Texas; Chicago, Illinois; Elgin, Illinois; and Phoenix, Arizona.
You may submit Form G-1145 with the following forms:
EOIR-29, Notice of Appeal to the Board of Immigration Appeals from a Decision of a DHS Officer;
I-102, Application for Replacement/Initial Nonimmigrant Arrival-Departure Document;
I-130, Petition for Alien Relative;
I-131, Application for Travel Documents, Parole Documents, and Arrival-Departure Documents;
I-140, Immigrant Petition for Alien Worker;
I-212, Application for Permission to Reapply for Admission into the U.S. After Deportation or Removal;
I-360, Petition for Amerasian, Widow(er) or Special Immigrant;
I-485, Application to Register Permanent Residence or Adjust Status;
I-485 Supp A, Supplement A to Form I-485, Adjustment of Status Under Section 245(i);
I-589, Application for Asylum and for Withholding of Removal;
I-601, Application for Waiver of Grounds of Inadmissibility;
I-601A, Application for Provisional Unlawful Presence Waiver;
I-694, Notice of Appeal of Decision Under Sections 245A or 210 of the Immigration and Nationality Act;
I-698, Application to Adjust Status from Temporary to Permanent Resident (Under Section 245A of the INA);
I-765, Application for Employment Authorization;
I-800, Petition to Classify Convention Adoptee as an Immediate Relative;
I-800A, Application for Determination of Suitability to Adopt a Child from a Convention Country;
I-817, Application for Family Unity Benefits;
I-821, Application for Temporary Protected Status;
I-821D, Consideration of Deferred Action for Childhood Arrivals;
I-824, Application for Action on an Approved Application or Petition;
N-565, Application for Replacement Naturalization/Citizenship Document; or
N-600K, Application for Citizenship and Issuance of Certificate under Section 322.
“