Looks like I'm in the same boat with you, Andrew, but in a different version. The classic AIM 5.9.6089. I was one of the people who saw the updates for AIM 7 Jan 1st, 2007 and never upgraded. Call me stuck in my ways but I'll never see AIM for anything except 5.9.6089 (and lower).
Unfortunately for me, downgrading my OS isn't a possibility, and even if it were, I'm not sure if it'd even help this version in particular.
Like you said, Andrew, I'd considered this another of AOL's many stunts (how many times did they say they were discontinuing it? I've lost count) but after googling and finding this thread, I sincerely doubt it.
If anyone finds this thread or knows how to hack AIM 5.9.6089 to continue connecting post-March 15th, I'd be very grateful.