As noted by a support document issued in June, Apple ceased iChat support for AIM users with legacy IDs on June 30, saying upgrading to OS X 10.7.2 or later would offer continued access. The company's new requirement, however, has seemingly ended legacy login support for all OS X versions, including 10.9 Mavericks and beta builds of 10.10 Yosemite, reports AppAdvice.
Using a me.com account registered with AIM, AppleInsider has confirmed operation in Apple's Messages app for both Mavericks and Yosemite has been rendered unusable. It should be noted that Apple is not planning to drop support for AIM itself in current and future OS X versions; just accounts attached to legacy mac.com and me.com IDs, and only for Macs running OS X 10.7.1 or below.
As noted in a thread on Apple's Support Communities forum, users have been receiving messages, like the one seen below, to upgrade to the latest OS X version to continue using AIM with mac.com and me.com IDs. The generic warning popped up on Macs running legacy OS X versions, as well as current the most current builds of Mavericks and Yosemite.
Aside from from these warning messages, Apple did not officially announce the change in service, likely leaving affected users confused as to why their accounts are suddenly failing to connect.
Source: Apple Support Communities forum user Ralph Johns
In addition to first-party iChat and Messages apps, users of third-party clients like Adium are also unable to log into their AIM accounts due to the new measures. Adium has since made users aware of the problem.
It is unclear how many Macs running up-to-date versions of OS X are affected by the issue and Apple has yet to offer comment on the situation.
Users looking for a quick fix can simply register a new AIM account with a current Apple ID through AOL's website, though buddy lists and other settings will have to be reconfigured in iChat or Messages on all affected devices.