The Answer - What has happened on this alt is a disease of Magic. Metaphorically it's like a virus that feeds on Magic.
Magic items can carry this contagion and the magic keeps the disease alive. When the disease contacts a person, it infects them and then uses magic energy to reproduce itself, killing the magic user. It works on arcane, natural and divine magic.
The higher the caster level of the victim, the worse the disease hits them. Fever, malaise, wasting, death. Clerics got ahead of it for a while, but their own magic fueled the outbreak and killed them.
People with no magical skills or abilities were also vulnerable, if they handled contaminated magical devices.
The disease cannot survive where there is little or no magic, so mundane objects cannot transmit the disease and if quarantined, become safe after about two weeks of being away from all magic.
Along a great wall there is a big painted message in Common, "The Lich Suuri Stratoe Necros has the answer."
The Lich was skilled enough and had enough time to ferret out the cause of the disease. He tried to call together a collection of agents to conduct the proper actions to begin to rid the wold of the disease. But he was consumed by the disease before his plan could be put into action.
His plan involved collecting a batch of magical artifacts and getting them to a certain place at the right time. In the difficult to reach place, with the right unique artifacts, an energy can be created that is magical, yet inimical to the disease, the artifacts would then have the ability to destroy the disease in any person or object by touch.
This plan lies written on a scroll on Suuri Stratoe Necro's desk. Find the Old Lich's lair, get through what's left of mundane traps and it'll be right there for the doing. All agent dispatched to recover it were killed in the attempt.
With the loss of a plan and the death of magic users and magical people, humans and others fled the old cities and founded new ones, being careful to avoid and outlaw all magic. Messing with such things is hot death and people who so mess are not especially welcome.
Looting old ruins now is much less fighting monsters and more CDC contamination control mixed with archaeology.