Has the ATF appealed the decision in Mock v Garland? They still list the factoring criteria on their website. Seems to me that they must be appealing, otherwise they should remove the rule and update their site to include guidance to people who registered their FreeBR. I agree with those here that say it's registered regardless of the rule and thus legal to put a proper stock or VFG on, but I think the ATF would owe a duty to citizens to also inform them that per the court's ruling, the firearm is not considered to be governed by Title 2 if configured without a stock or VFG. I think that logically that must be the case even though conventional wisdom is that a SBR cannot return to being a pistol but could return to being a Title 1 rifle due to the fact that if left unchanged with a brace, it was never made as a rifle, SBR or otherwise.
caveat: I'm not a lawyer, YMMV, and I intended to actually use the tax exempt pistol braced registrations as SBRs, so this is purely a thought exercise for me.