It was in an ATF letter... maybe June or July of 2019. Made my TAC-14 folding adapter illegal to mount on my firearm (which is going to be AOWed so I can install it and have OAL of 24”; not a stock, so I can go back to the firearm status, if desired).
Prior to that, firearms and pistols were measured with the brace unfolded, since that was how rifles were measured. But if you remove 8” to 10” of length... you move away that much more from a pistol being 26” OAL... which was likely the reason for the “change.”
If it is a folding tube, from how the letter was written, I’d say it would be measured folded. But definitely look it up and check for yourself.
In regards to the inference about all of us running just buffer tubes in the future... speak for yourself. I didn’t roll over for bump stocks, even if I viewed them as useless... and I’m not rolling over for braces. If they want to make them illegal without any due process, come and get them.