It should be no surprise that the ATF has pistol braces in their sights. In December they released a proposal for regulation and then retracted that once public comments were received. One of the problems with the prior proposal is that there were no specific criteria for determining what is a pistol with brace and what is a rifle with pistol brace redefined as a stock.
They've got a new worksheet with a point system to verify what is and what is not a brace. If enacted it will likely eliminate most pistol braces and braced firearms. There was a mention back in December of waiving the NFA tax when registering the pistols as SBR's. That's out the window too now.
You can read the full proposal at: https://www.atf.gov/file/154871/download
You can also see more info along with contact info: https://www.atf.gov/rules-and-regulations/factoring-criteria-firearms-attached-stabilizing-braces