Not collapse: recycling. In a steady state population, you need to be mining enough new metal to replace items lost to use or rust. If the population drops, people just start using dead uncle Harald's knives when theirs break.

In such a situation, you'd actually expect expensive new production to drop all the way to zero until the recycling buffer was used up. The effect would be superlinear with the death rate, not proportional.

Maybe so. But maybe the recycling argument doesn't apply so much for lead as for iron.

It applies more so for things I've dealt with. Iron rusts while lead doesn't. Flashings and windows etc that use lead last a very long time, iron doesn't.

The contention in the linked article was that atmospheric lead was the result of metal smelting. They're measuring the mining industry as a whole, not lead production.

Whatever they say, they're measuring production of lead and other metals that occur with it. Silver, copper and zinc would be the main ones, I think. Not iron, however.

Perhaps centers of industrial production are more prone to damage from pandemics than rural areas meaning that mines and factories will be gutted out while farms and villages are maybe less damaged?

Farms and villages just get abandoned, decay, and are lost to time.