Ok, the answer is no. You do not have to register suppressors with the ATF. Why is this relevant. Well, because from a statutory perspective only mufflers and silencers are NFA firearms. And even though every manufacturer uses the term suppressor, they are all registered as either a silencer or a muffler.
And the same thing goes for vertical grip or angled grip. These terms are not defined within statute. All that matters is whether or not your pistol has a single short stock at an angle below the line of the bore. What is a stock then? It's not defined but anecdotal evidence suggest it's anything that can be gripped with one hand using at least one finger and a thumb. That is a short stock and you're only allowed to have one on any pistol less than 26" OAL.
Grip is irrelevant. Angled is irrelevant because even a vertical grip is at a 90% angle below the line of bore. People are confused because they think a manufacturer label means something. It means NOTHING.
The Magpul AFG is legal because someone at ATF decided it isn't a short stock. And that's the only reason it's legal, PERIOD.
And until someone at ATF says the Fortis Shift is not a short stock, it will remain a sketchy item to install on a barrel shroud (the only statutory term for a handguard) of a pistol less than 26" OAL. Why is it sketchy? The picture above and the video I posted are self explanatory. All it takes is a one finger, one thumb grip to be a stock.