Other times, playing online is made impossible due to DNAS refusing to authenticate a specific title (in the case of NFL Blitz Pro & NHL Hitz Pro) and while rare, total server discontinuation (such as Risk: Global Domination and, most likely, Deer Hunter). In other cases, it is simply a matter of false reporting, as was the case for FlatOut & FlatOut 2.
Compiled with information from various forums, and the Wikipedia List of PlayStation 2 online games, this is a complete list of all PS2 online games maintained by GameSpy that have previously been reported as having their online services terminated.
Thanks for the input, everyone. We can now safely assume that all PS2 online games hosted by GameSpy are still online and functional, EXCEPT for the following games with special conditions:
All-Star Baseball 2005 - (FIXED)
Battlefield 2: Modern Combat - (GameSpy ID Account Creation FIXED)
Blitz: The League - (FIXED)
Deer Hunter - (FIXED)
MLB SlugFest: Loaded - (FIXED)
Mortal Kombat: Armageddon - (FIXED)
Mortal Kombat: Deception - (FIXED)
NBA Ballers - (FIXED)
NBA Ballers: Phenom - (FIXED)
NFL Blitz Pro - (DNAS Error (-103) This software title is not in service. Unable to connect to the online server because of this.)
NHL Hitz Pro - (DNAS Error (-103) This software title is not in service. Unable to connect to the online server because of this.)
Risk: Global Domination - (FIXED)
Tony Hawk's Pro Skater 3 - (FIXED, via THPS.US and BobzEnt.info servers)
Tony Hawk's Pro Skater 4 - (FIXED, via THPS.US and BobzEnt.info servers)
Tony Hawk's Underground - (FIXED, via THPS.US and BobzEnt.info servers)
Tony Hawk's Underground 2 - (FIXED, via THPS.US and BobzEnt.info servers)
Tony Hawk's American Wasteland - (FIXED, via THPS.US and BobzEnt.info servers)