While I don't know if HBD will actually depeg on external markets when the debt limit is hit (30%) the HBD haircut rule will come into affect. The internal market might fair better due to HBDstablizer

This basically means HBD from posts will stop printing, I think Conversions either also stop or result in vastly lower conversion rates.

I'm not 100% sure what happens to savings interest and DHF payments been a few years since I've last thought about the debt limit system.

All in all it's not a good thing to happen, but I honestly don't think we'll get there any time soon. Largely because hardfork 28 on the 19th of november will significantly (due to low price) cut inflation by taking the HBD in the DHF out of the inflation calculations, since it's 'dormant' until it's payed out.

Similar to what was done when removing DHF from the debt calculations.