US airline trade group Airlines 4 American confirms that it has “approved a new international standard that will allow for ‘unspecified’ and ‘undisclosed’ in addition to ‘male’ or ‘female.’”
Effective June 1, 2019 this accommodates passengers who have an ‘X’ on their IDs for gender. This is currently an option for state IDs from Arkansas, California, Colorado, Minnesota, Oregon and Washington, D.C.