Will the steep beta descent mode become illegal for new aircraft?
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?