I have believed that no one has used demonreach for a while. with the idea that the heavy hitters of the fallen not knowing what it was when they used it and shagnasty not having an issue showing up at a place for a trade where a bunch of his brothers were being held makes me believe that the last time it was used was before that ice age
I hate to put this idea out there, because time travel complications can be so annoying, but here goes.
What if Shagnasty didn't have a problem going to Demonreach Isle because he had no clue any of his kind were imprisoned there; because, outside of the prison, they haven't been captured yet? That will happen at a future time. For all we know Harry will be the one that captures them; and to take this idea to its ultimate conclusion, Shagnasty could be one of the prisoners.
Whatever the case may be, I have no doubt that at some point we are going to see Harry capture and imprison a supernatural being. Actually, I think it will happen twice. The first time will show us the process and how difficult it is to pull off, the second time will probably occur in the BAT, and Harry will probably have to imprison multiple supernatural entities.