Resistance Retribution will no longer be supported by online services as of May 5.
Sony has elected to discontinue support for the PSP title, which released in 2009.
Update: Online services for PSP title Resistance Retribution will be discontinued as of May 5th, 2015. Thanks for playing!
— PlayStation (@PlayStation) February 10, 2015
It may come as a surprise to the handheld's detractors to consider the fact that the PSP shooter currently supports up to eight players in online multiplayer. With its retirement, 2012 Vita title Burning Skies will be the only actively-supported Resistance multiplayer game.
Ah well; it's had a good run. Six years is pretty decent, given the franchise's limited reach and the PSP's almost total sunsetting in the west.
The Resistance franchise seems to be on hold, with no announcements since Burning Skies. Insomniac Games has moved on, and Sony doesn't seem to have put anybody else onto it; it probably feels Killzone is enough in terms of first-party shooters.