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 tickets are still sold as if the flights are from/to Ataturk Airport. There are only a few domestic flights under the ISL code, which are already from/to the new airport. Thus I don't expect any question regarding the new airport on Travel SE for now, maybe just one.