From the Bungie.net web site:
we’ve decided to make the unlockable multiplayer map, Foundation, available to everyone... and wanted very much to share Foundation – inspired by our old shoot ‘em up Marathon – with Halo 2 players everywhere.
note: it was more than "inspired by" it actually *was* a direct port of Thunderdome that was later tweaked to give more central cover and work with Halo 2's physics. (The elevators were essentially player launchers)
The Halo 2 autoupdate unlocks the level for everyone.