NetherRealm confirmed that at least two PlayStation 3 fight sticks are compatible with the PS4 version of Mortal Kombat X. Confirmed so far are the Mortal Kombat Tournament Edition fight stick from 2011 and the Injustice: Gods Among Us fight stick from 2013. It's not yet clear what other sticks are supported, or if the Xbox One version will support Xbox 360 sticks.
Developer Lab Zero Games announced last year that it planned to support PS3 fight sticks on the PS4 version of Skullgirls via a custom USB driver. Lab Zero also said it would make its driver available to other developers at no cost. NetherRealm didn't disclose whether or not it was using Lab Zero's driver, but we've inquired with the Mortal Kombat developer for details.
Mortal Kombat X will be released for PlayStation 4, Windows PC and Xbox One.