Oh, a perfect example of a question that forces me to look into the code and learn, that what I thought was certain, I actually remember wrong :o)
The description I've posted here is still valid, although links to code no longer point to exact locations, since the code changed (I wonder if there is a way to keep such links valid even in case of changes; sure, I could link to specific commit, but that would mean I'd be pointing to potentially outdated code...). HF26 changed BOTH soft limits to 20% from 9% and 10% (this is where I remembered it wrong thinking upper soft limit is 30%) - meaning there is no smooth transition when less and less HBD is printed for comment rewards (now that I think about it I remember fixing a bug related to those limits being equal). Only the hard cap was changed to 30% from 10%. In other words, when we reach and cross 20% HBD debt, posts won't receive any HBD (that portion of reward is paid in liquid HIVE), conversions from HIVE to HBD with collateralized_convert_operation are also completely disabled. Payments from proposals as well as interest on HBD savings are not affected - they are still paid in full with HBD. Only if the debt continues to grow (due to falling price of HIVE or over time from still active sources of new HBD) we can reach 30% hard limit where HBD loses its dollar peg.
A sharp cut for the authors rewards ... I wasnt expecting that. Interesting about the hive to hbd conversions. About the dhf oayouts and the interest, they can be basicly set to zero if the top stakeholders and witnesses decided to.... so they are also regulatable so to speak.