Syphon Filter originally launched for PlayStation on February 17, 1999, followed by Syphon Filter 2 on March 14, 2000. Syphon Filter: Dark Mirror originally launched for PSP on March 14, 2006, followed by Syphon Filter: Logan’s Shadow on October 2, 2007.
The only two games in the series that were not rated are Syphon Filter 3, originally released for PlayStation on November 6, 2001, and Syphon Filter: The Omega Strain, originally released for PlayStation 2 on May 4, 2004.
The titles will likely be offered as part of the PlayStation Plus Premium tier set to launch in June, which includes access to a catalogue of “beloved classic games” from the original PlayStation, PlayStation 2, and PSP generations.
Thanks for the tip, @The_Marmolade.