The Artifact pickup stayed but there was an altartrigger actor at least since august 1997, then it disappeared again past November. There was also TargetArtifact that expanded altartrigger. As far as I understand it, those were used as individual triggers for individual artifacts with different tags and what not.
Once all the artifacts were in place at the altartrigger, the Event of the AltarTrigger was triggered. It even says in comment "Altar full, so open it."
Also, the individual targetartifact triggers would gain the mesh and look of the artifact you just placed, so there would be a visual indication. I forgot if I made a test map for this or not and it might be broken on some of the versions but this is what it was supposed to do.