There is a lot of concern about the domain extension .io going away, because it is assigned to the British Indian Ocean Territory, and the U.K. recently handled over that territory to the country of Mauritius. And, worse, the official policy of ICANN is to phase out ccTLDs for countries that no longer exist.
But in this case, the territory still exists, it's just been transferred. So it is possible that .io just gets transferred to Mauritius. We will have to see what ICANN decides. Considering the number of domains on .io, I don't think it would be wise to retire it.
Meanwhile, back in the Soviet Union, after 33 years of no longer existing, there are still over 100,000 .su domains registered and accessible. And you can still register a new .su domain today.
But, perhaps that is because in Soviet Russia, you don't register a domain, the domain registers you.
