From One Mile at a Time: the main U.S. carriers have started Basic Economy fares that are highly restricted, including no advance seat assignments. As Delta expands it internationally, they and their partners Air France, KLM, and Alitalia will charge for the first checked bag:
Delta has eliminated the free checked bag for those traveling on basic economy fares between the US/Canada and Europe/North Africa. This new policy applies for tickets booked as of December 6, 2017, for travel as of April 10, 2018.
This is new for U.S. legacy carriers. The lowest fares have this restriction, while Economy with the some of the features previously expected is called Main Cabin, with a higher fare.