It's not so much that they dropped it. The way it worked with DSM2 is just not available with DSMX. I read a good technical explanation but can't find it.
This is from the Spektrum DSMX Addendum:
Following are the operational differences:
Brownout Detection - Not Available on DSMX Receivers
DSM2 receivers feature Brownout Detection that flashes the receiver’s LED if a power
interruption occurs. While DSMX receivers have QuickConnect and recover instantly
from a power interruption, the architecture of DSMX prevents Brownout Detection when
operating in DSMX mode.