Mortal Kombat 11 might be another game that has added crossplay support on PS4. An update was released today that mentions crossplay in patch notes.
Warner Bros. Interactive and NetherRealm Studios have yet to comment on the addition of crossplay support for Mortal Kombat 11 but a new patch was released on PS4. The patch notes as can be seen through the PS4 Dashboard mention adding crossplay. It is very unlikely that this is a mistake knowing how open is Sony to adding crossplay support on their platform.
Sony has finally allowed Microsoft to bring Minecraft Bedrock Version to PS4 which adds crossplay support for PS4. The update adding this support was just released recently thus uniting the various platforms under one roof. Mortal Kombat 11 is apparently the second game to get a crossplay support update this week.
There are no official patch notes but from what the players have discovered so far, the following changes are a part of this new update.
It is possible that crossplay support is an announcement that will happen at The Game Awards and will be revealed soon. In any case, fighting games rarely get crossplay support on PS4 although some of the major games have launched with crossplay like Call of Duty: Modern Warfare, it is still a slow process on PS4.
Mortal Kombat 11 is available for PS4, Xbox One, PC, and Nintendo Switch. There is no update released for PC and Nintendo Switch so it is possible that the crossplay support patch dropped than the expected announcement.