Assuming the British/American air base remains on some agreement with the Mauritian government, then the Chagos Islands may remain as a special territory of Mauritius, justifying the continual existence of an ISO 3166 code for it.
It's "Exceptionally reserved" "at the request of International Telecommunication Union (ITU)".
Although my gut feeling is that if the deal happens, Mauritius would probably ask to exceptionally reserve `IO` as well. Still, I'm not sure if I'd risk using a `.io` domain since it's not guaranteed.