I just saw this presentation at the Chaos Computer Club conference, for an “Ethical Hardware Kit with a PCB microcontroller made of wild clay retrieved from the forest in Austria and fired on a bonfire. Our conductive tracks use urban-mined silver and all components are re-used from old electronic devices”. It was part of the feminist hardware strand!
AFAIK, urban mining is less about some post apocalyptic contingency, but rather, as the name implies: Sourcing material from any urban environment. This can include recycling, but also stuff like using material of demolished buildings, etc.