They have been used to route out copper from plated PCBs, cut out the part that etchant would normally remove.
I have not seen that appleied conductive material to bare material (such as fiberglass sheet) to make a PCB.
Before PCBs, we used to have PWBs - where actual wire was put down for the traces. Like in the 80's.
Glueing double sided boards together with B-stage material in between has been the norm for quite a while.