It turns out Netherrealm Studios is adding a region lock to the multiplayer of Mortal Kombat X. This was confirmed in a tweet by the executive producer Shaun Himmerick, who wrote: "region locks will be the same as injustice and MK9 were".
That means that at least players of PlayStation 3 and 4 won't be able to play against opponents from other parts of the world. Microsoft previously didn't allow this limitation, and it remains to be seen if Xbox 360 and Xbox One will be affected by this decision.
The tweet was removed when it started to get attention, but fortunately the Mortal Kombat fansite Testyourmight.com managed to get a screengrab as proof.