Killzone: Mercenary is one of the most notable PlayStation Vita exclusives. It had a multiplayer and it was quite popular. “Was” because Sony closed the game’s servers at the end of last week.
The company usually warns players in advance when it plans to shut down the network services of its projects. For example, the Driveclub community was given a whole year to say goodbye to the game. The PlayStation site has a separate page that describes which servers have already closed, and which will be closed in the near future.
However, Killzone: Mercenary is not marked there. You would think that the servers have gone to scheduled maintenance, but Sony technical support confirmed in a conversation with a Reddit user that the multiplayer was turned off for good.
Mercenary has a single-player campaign, so the shooter will continue to work. Unfortunately, some of the trophies in the game are connected to the multiplayer.