Will the steep beta descent mode become illegal for new aircraft? The PC6 is a popular skydiver carrier due to its ability to use steep nose down beta descent to get back to the ground quickly, even beating free falling skydivers. 


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FAA AC25.1155x (I only saw the draft) proposes that a "means to prevent intentional or inadvertent selected of reverse thrust or propeller pitch below the flight regime" be required, which cannot be overridden. The intention is to prevent several accidents resulting from said deployment. The lockout means must not degrade landing performance, so the intent is not to eliminate beta entirely. 

Does this mean that no new aircraft can be certified that has a descent mode from altitude similar to the PC6?
I wouldn't call it a "descent mode from altitude".  It's considered to be a dangerous and foolish practice to operate an engine in ground beta (generally coming back into DISCING - blades flat) while in flight to get steep descent rates.  If the prop doesn't want to come out of ground beta, you are dead meat.  

When the Twin Otter was in production, it was an open secret that demo pilots were using DISCING to get crazy steep approaches to impress customers.  After a couple exciting events, flight ops finally put an end to it.  

Some airplanes already have ground beta lockout systems.  Is this a proposal to mandate a system for the PC6 specifically?