Netherrealm Studios quietly updated Mortal Kombat 11 this morning to add “Krossplay”, which is exactly what it sounds like. The feature is currently in beta, so there may be some issues but for now players on Playstation 4 and Xbox One will be able to play against each other in Kasual matches online.
Unfortunately, there are some caveats. For now, the feature will not allow you to play in Ranked mode. Similarly, it seems Nintendo Switch, Google Stadia and PC players won’t be able to join in on the fun either. Finally, it seems that all your progress (such as unlocked skins, variations, and kustomisations) will be exclusive to your primary console. So progression does not follow you across all platforms.