Sort of weird people are still talking about this like it is a mystery TBH.
Same thing that happened to Frank Mir's interim belt and Whittaker's interim belt and every other interim belt in the universe. It ceased to exist when it was changed to a unified belt. All the stupid ass Conor BS garbage aside, Dana said repeatedly that the Khabib fight was for the real belt. The Khabib fight happened/Tony pulled out/Khabib won/he has the unified undisputed belt. So once that belt exists the interim belt doesn't. From Dana announcing that fight was the real title right up until Conor got stripped the whole situation is pretty straightforward, yet people are talking about how weird it is Tony doesn't have a title anymore?