Back on December 23rd, of 2020 in a dramatic reversal, the deputy director of the ATF Marvin RIchardson, signed a withdrawal of the guidance, only five days after the guidance was published. The guidance was withdrawn pending further Department of Justice review. Here is the concluding paragraph from atf.gov’s document:
Upon further consultation with the Department of Justice and the Office of the Deputy Attorney General, ATF is withdrawing, pending further Department of Justice review, the notice and request for comments entitled “Objective Factors for Classifying Weapons with ‘Stabilizing Braces’,” that was published on December 18, 2020. 85 FR 82516. As explained in the notice, the proposed guidance was not a regulation. The notice informed and invited comment from the industry and public on a proposed guidance prior to issuing a final guidance document.
The withdrawal of the guidance does not change any law, regulation, or other legally binding requirement.