Juli Clover, writing for MacRumors:
Apple recently removed older versions of OS X and other discontinued software from the Purchased tab of users who had previously purchased or downloaded them. With the change, it is no longer possible for users to download Aperture, iPhoto, OS X Lion, OS X Mountain Lion, and OS X Mavericks from the Mac App Store.
It is not clear if Apple's decision to prevent users from downloading older software from the Mac App Store is a temporary bug or a permanent change. The software has, however, been unavailable for several days now.
Not sure if this is intentional or not. If so, it's a very baffling decision, and reinforces the fear that digital software you paid for can be gone in an instant at the whims of the people who control the pipe. Not a good precedent to set.