https://www.justice.gov/opa/pr/just...s-stabilizing-braces-accessories-used-convert
The rule goes into effect on the date of publication in the Federal Register. The rule allows for a 120-day period for manufacturers, dealers, and individuals to register tax-free any existing NFA short-barreled rifles covered by the rule. Other options including removing the stabilizing brace to return the firearm to a pistol or surrendering covered short-barreled rifles to ATF. Nothing in this rule bans stabilizing braces or the use of stabilizing braces on pistols.
From the ATF, a quick reference sheet
The rule goes into effect on the date of publication in the Federal Register. The rule allows for a 120-day period for manufacturers, dealers, and individuals to register tax-free any existing NFA short-barreled rifles covered by the rule. Other options including removing the stabilizing brace to return the firearm to a pistol or surrendering covered short-barreled rifles to ATF. Nothing in this rule bans stabilizing braces or the use of stabilizing braces on pistols.
From the ATF, a quick reference sheet