It came, then it disappeared, but now it's back again. As you already surmised from the title, the Android 4.3 update for the AT&T variant of the Galaxy S4 is now back in circulation and making its way to users of the device over the air. AT&T doesn't mention what issue caused the update to be pulled last time, but then again, as long as users can start upgrading to the much-awaited new version of Android once again, we shouldn't be hearing any complaints on that.
As a refresher, the Android 4.3 update adds Galaxy Gear compatibility, faster performance and improved RAM management, new camera firmware and modes, KNOX security support, and more. AT&T suggests to check for the update in the Settings » About phone » Software updates menu, though as always, you might have to wait a couple of days before the update shows up on your device.