It's certainly a thing for the payment rail - Bitcoin qua payment rail is entirely irreversible. So as a category in the real world, it is trivially a thing.

Whether such storage exists is a much better question. I think it depends on what you want to do with it. For example, if the Bitfinex guys were ideologically motivated, they could've spent all the money on, I don't know, political ads or whatever. More generally, I think you can certainly come up with storage that is outside the reach of a given threat actor in pursuit of a given goal.