HBD->Hive conversion existed before the HF. This conversion is no longer a pegged conversion after the haircut line is reached (in other words, you get less than $1 of Hive for 1 HBD at the haircut level, this was also true for a long time. So protection against a death spiral has been around for a long time.

What was added in the HF was a conversion in the opposite direction (Hive->HBD). This conversion was created to enable Hive holders to prevent speculators from driving up the price of HBD too far above the peg value (because Hive holders can create new HBD for just $1 and sell it for a profit if HBD goes above 1.05). And we've since seen that this new mechanism was very effective at preventing a big upside move of HBD.