They won't be illegal period, ever end of story. ATF does not pass laws, they do not legislate. It can't stand up in court. You can't say one thing on one day - it's ok it's not an SBR go ahead and manufacture it, then after 4 million are on the streets say - oh, that's illegal now. If Biden had put hundreds of judges on the bench, he might get away with it. But Trump put those conservative judges there, not Biden and they won't stand for it.
You live in Texas a state where the Governor will before this so call "rule" is ever proposed sign into law a Sanctuary Law that will declare Texas a 2nd Amendment Sanctuary State similar to what Arizona has done. State and Local law enforcement, Sheriffs, etc will all be directed to
NOT enforce Biden's illegal rules. And there isn't a thing they can do about it either because they are the ones that set the precedent with Sanctuary Cities.
The Sheriff here in Collin County has gone on record numerous times stating that he will not enforce any illegal laws that undermine the 2nd Amendment.
So unless you have done something else to get federal authorities on your case, no one will enforce it. Believe me, the ATF and FBI will have enough on their hands over the next 4 years dealing with other things that happen as a result of open borders than to worry about enforcing rules on pistol braces in states where the authorities will not.
Those that panic and sell such guns are doing exactly what they want. They WANT people to be scared into getting rid of them. They want them to be removed from the market as far as manufacture and selling is concerned (and they will I'm sure manage to restrict manufacture/distribution until the ruling is overturned I'm sure).
There is no way in hell that the idiot that he nominated to run the AFT (Biden's stupid name for the ATF) will pass Senate confirmation. It ain't gonna happen. There is a reason that there hasn't been a Senate confirmed head of ATF in 7 years, no one can get confirmed! You've already seen one Senate Confirmation not happen because of past comments a nominee has made, wait till they try with him. He deleted thousands of online tweets and messages already but copies are out there just as they were with Tanden - not to mention his comments that people should be arrested BEFORE they commit a crime, that we store our guns until the zombies attack, that the Branch Dividians shot down FBI helicopters at Waco, etc. I could go on and on about that fool.
The downfall (beauty) about Executive Orders (or ATF "rulings") are that they can be undone as easily as they were done. Even Biden in his senile stupidity knows this. He knows he doesn't have the votes to get anything thru the Senate in regards to gun control, so this is what he's doing to placate the gun control nuts that make up his base.
They are not illegal now...but most likely will be soon...until it gets to the courts. Once it hits the courts, who knows. Can't say we didn't expect this. Total BS.