I'm not a lawyer, nor do I play one on TV ......... It is my opinion that owning a pistol brace and/or bump stock is not illegal, if they are not attached to a firearm. The legality comes to play when you physically attach said item to a firearm.
If they become illegal at a certain date, then they can not be purchased legally after that date. However anyone owning one prior to that certain date "should be" grandfathered in and allowed to legally own and use said brace/bump stock, due to the fact the item was purchase when they were still legal to purchase.
If a law comes into effect that makes possession of such item illegal, regardless of date purchased, that becomes tantamount to confiscation. And according to the Constitution of the United States of America, the government can not seize your property without just compensation.
So if you buy one now, while still legal, you "should be" grandfathered in or the government will have to reimburse you for the value of the brace. Just my opinion.