The IATA code IST and the ist tag are currently used for Istanbul Ataturk Airport. This airport will be closed on 31 December 2018 and its IATA code will be transferred to Istanbul Airport, which is temporarily using the IATA code ISL.
If we use the ist tag for the new airport as well, we won't be able to distinguish these two airports. I wonder if we have any solution for that.
Edit: The transfer has been postponed to March 2019.