Apple released this Document a little while ago now.
It's title is:-
OS X: Using me.com or mac.com IDs with AIM in iChat requires OS X v10.7.2 or later
The short version seems to be that as Apple IDs they will no longer be working AIM valid Screen Names after the 30th of June 2014
Previously any old @mac.com ID would work as a AIM login whether it had originally been a Paid for Account, one that had Lapsed or the Trial Account during the .Mac period or as an "iChat Name" one that was registered during the MobileMe days.
@me.com names only worked whilst they were paid for, the period your trial was active or at the cross over point with iCloud where they were iCloud issued Names (were "converted" to iCloud).
To work as AIM valid Screen Names the password with Apple had to comply with the 16 Character limit with the AIM Server.
At iChat 6 the App and the names (mainly iCloud linked ones) were subject to the app logging in to both AIM and Apple as a means of verifying the Password.
This continues in Messages.
This did mean that the @mac.com names worked in iChat 5 and earlier as they did not have this iCloud double check as it were.
@me.com and @icloud.com names linked to an iCloud account have never worked in iChat 5 or earlier.
It appears the Apple IDs in question will still function for iMessages, FaceTime and other places you might used - even if not linked to an iCloud account.
It is only as an AIM valid Screen Name that it will stop working.
AIM have been sending reminders to your AIM Screen Name in Messages
The Document implies but does not state that these IDs will work in later version of the OS.
However @mac.com names have never been subject to this double check Login so it seems likely they will fail there as well.
(Same applies to any old @me.com that have never been linked to iCloud.)
It seems that this will standardise which IDs will work and which will not across all version of iChat and Messages to some extent.
The issue being with earlier versions or the OS/ichat that cannot use iCloud issued names and need to register for AIM names instead.
10:26 pm Saturday; June 28, 2014
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
Couple of iPhones and an iPad