I received this message from Turkish Airlines yesterday:
Dear Esteemed Customer,
The Great Move is beginning! We would like to kindly remind you that as of April 6th, 2019, 14:00, all our flights planned for Ataturk Airport (IST) will be conducted from our new home, Istanbul Airport (IST). Have a nice flight!
For those of you flying soon, please let us know what the new airport is like and what type of transportation you took into the city.