Regardless of that, it is very stupid the short barreled rifle NFA registration. I think if you look at FRT triggers, the auto key card, bump stocks and the brace they have one thing in common: the AR15 has been the focus. I would also suspect that in spite of mass non compliance people will be prosecuted and made an example of. Good examples, the Twitter meme artist who was thrown in prison, the people who entered the Capitol on January 6th 2020. Got more than one brace? You get to the top of the list if you didn't register them.
You ever look at the number of prosecutions for weapons violations? NFA related violations barely make a blip on the radar from what I can find.
"While there are numerous federal provisions, just one of these accounts for three out of every four federal weapons prosecutions. During FY 2017, federal records indicate this was the lead charge for 6,161 prosecutions. This statutory provision is found at 18 USC § 922(g)(1) which prohibits any felon from possessing, shipping/transporting, or receiving any firearm or ammunition. The penalty for violating this law is up to ten years imprisonment and/or a $250,000 fine" source:Federal Weapons Prosecutions Rise for Third Consecutive Year