ISO 3166-1 defines codes for "countries, territories, or areas of geographical interest". When the country is no longer a country, the country it's becoming part of might very decide to treat it as something still deserving an ISO code and thus a ccTLD. (and such a status makes sense for pure geographical reasons, its >2000km from Mauritius)
I believe the reasoning is that the list was originally used for post, so far-flung regions of a country may have their own codes, even if they're not politically separate. GF, French Guiana, is a good example. Politically, it's merely a region of France, but it still gets its own code.
If Mauritius decided to used a variant of Chagos/Chagas (so probably CS, since that's the only available code that still somehow fits) then IO will probably be ejected from ISO 3166.
