I believe it's probably only because of the woefully under-developed state of cryonics itself that the practice of voluntary death through cryopreservation (cryothanasia) haven't been seriously researched: rather counter-intuitively, cryonics companies are too few and mostly have enough trouble on their hands to bother disrupting the status quo.
Getting frozen before you die can well be problematic, but not necessarily impossible in all jurisdictions. I believe it's just not well researched. Cryonics has low demand as it is, and cryothanasia requires even greater mental effort to voluntarily choose death, before dying the 'natural' way, and make all the necessary research and preparations yourself, so I wouldn't be surprised to learn that noone bothered yet, or noone who made their efforts public, at the very least. Which doesn't mean that this is impossible. Suicide tourism is a thing, after all.
I would recommend directly contacting Danila Medvedev (medvedev@tranhuman.ru) from Russian company CryoRus, if you are really practically interested in the prospect of cryothanasia - it is likely that it is possible, but noone is going to offer it as a product, so far, you will have to research it for yourself; they are at least entertaining the idea.
BBC article
I'm sure I'm not the only one who greatly admired him. The theme of his stories was progress; they were set in a fantasy world, it's true, but one that was frequently a direct analogy to our own past, and where the golden age was always right now. The recent books made this ever more obvious.
We have lost a great man today, but it's the way he died that makes me uncomfortable. Terry Pratchett had early-onset Alzheimer's, and while I doubt it would have mattered, he couldn't have chosen cryonics even if he wanted to. He campaigned for voluntary euthanasia in cases like his. I will refrain from speculating on whether his unexpected death was wholly natural; whether it was or wasn't, I can't see this having a better outcome. In short...
There is, for each of us, a one-ninth chance of developing Alzheimer's if we live long enough. Many of us may have relatives that are already showing signs, and in the current regime these relatives cannot be cryonically stored even if they wish to try; by the time they die, there will be little purpose in doing so. For cryonics to help for neurodegenerative disorders, it needs to be applied before they become fatal.
Is there anything we can do to change that? Are there countries in which that generalisation is false?