The ATF Ruling 2021R-08F, expected to be published today (January 16th 2023), redefined pistols with a stabilizing brace a Short Barreled Rifle. 

Here are your options if you have a pistol with a brace:

You'll have 120 days from the date the ruling is published to register the pistol as an SBR (free). See below for further details.

OR

Remove the brace and make it impossible to be reinstalled. 

OR 

Forfeit the pistol in to your local Police Department

OR

Replace the barrel, longer than 16".

OR

Destroy the firearm

From the ATF's website:

"The rule outlines the factors ATF would consider when evaluating firearms equipped with a purported “stabilizing brace” (or other rearward attachment) to determine whether these weapons would be considered a “rifle” or “short-barreled rifle” under the Gun Control Act of 1968, or a “rifle” or “firearm” subject to regulation under the National Firearms Act. The rule’s amended definition of “rifle” clarifies that the term “designed, redesigned, made or remade, and intended to be fired from the shoulder” includes a weapon that is equipped with an accessory, component, or other rearward attachment (e.g., a “stabilizing brace”) that provides surface area that allows the weapon to be fired from the shoulder, provided other factors, as listed in the definition, indicate the weapon is designed and intended to be fired from the shoulder."

If you're registering a pistol you built, you will need to engrave your name and the location where the pistol was manufactured into the lower prior to filing the Form 1. Manufacturers of destructive devices must engrave certain information including the manufacturer's name and location. 

In other words, if you purchased a brace and installed it yourself, you must engrave it in accordance with 27 CFR 478.92 & 479.102. If you purchased a pistol with a stabilizing brace installed by the factory, you can adopt the manufacturer's markings.