.su is administered by the Russian national registry, because Russia is de facto the Soviet Union’s successor state. In this case, though, would .io stay with the UK or come under Mauritian control? It’s not clear.
IMO what will probably happen is that ICANN "promotes" the zone to being yet another top-level non-country code domain like .biz or .horse etc. Which is effectively what it is now.
Edit to add:
I don't think the .su precedent is applicable here. The Soviet Union was an internationally recognised state with a population, military, Montevideo Convention duties, seat at the UN, etc. The BIOT was and is nothing like that.
Sure, there's a lot of evidence that they were "terra nullius" before being claimed for the British empire. But the Chagos archipelago was inhabited utill its population was compulsorily expelled in the mid twentieth century.
I was surprised to discover that Ascension even has a ccTLD. I guess I assumed that the population was wholly military.
That's because it was created by the same guy who created .io and .sh - British DNS "pioneer" Paul Kane, who clearly had a passion for finding remaining corners of the British Empire that could "claim" a bit of internet land (for his own profit).
ICANN will act in whatever manner causes them the least trouble, which will be to retain the status quo. They have absolutely no incentive to behave otherwise.
It's pretty detrimental to relationships when one side pre-declares their rules and policies, and then arbitrarily decides to ignore them.
I'd say they have a lot of incentive to behave in whatever way their published policies state, regardless of the impact to users of the cctld.
(I don't know what the policies are)
Before the retirement of .yu, Slovenia wanted to hold on to it, but it was not the successor state of Yugoslavia so they had to relinquish control and pass it to Serbia. So going by that logic, it would not stay in the UK (for long).
It’s like if Guantanamo Bay had its own ccTLD.
The land will go to Mauritius, the legal entity of British Indian Ocean Territory will cease to exist (presumably).
Mauritius could decide to incorporate it as "Mauritius Indian Ocean Territory", hence maintaining the CC. I expect .io owners will likely suggest something like that, while showing them how much money they could get from a 10-15% deal similar to what Tuvalu has for .tv. Nobody likes to burn money.