Exactly the same thing that used to be behind here before the server upgrade (ie XOOPS).
However, this site was significantly older than either of the other two and locked into an obsolete version of XOOPS that couldn't be readily upgraded. When the hosting company set a deadline to upgrade to PHP5, that was a death warrant to the version of XOOPS that was in use. There are newer versions of XOOPS, but they are no more IBrowse friendly than vBulletin is.
Even if AO could have colo'd its own box (some places are pretty cheap, $100/mo sans management,) I believe the security implications of running PHP4 and older XOOPS would be too large to be worth it.
People tend to not realize that older technology is often abandoned for security issues and not simply functional issues. For instance, the Windows 9x kernel is dead and no longer supported because in the face of modern cyber-warfare, it is worse than Swiss cheese and does not have the internal framework necessary to provide the hefty security needed to survive.
One would not mount a deadbolt on a straw door. Sure, one could put a reinforced backing on the straw door, but the frame would no longer support the door, nor the house support a reinforced frame, and so on. At some point you just need to replace the whole thing.
Bear in mind, while it does lack important features such as protected memory, I speak not specifically to the Amiga. More so, I speak to the Amiga-era frameworks and applications, such as PHP4 and XOOPS, which had to be moved beyond.